[Fix rubocop#279] Add new Minitest/NonExecutableTestMethod cop

Resolves rubocop#279.

This PR adds new `Minitest/NonExecutableTestMethod` cop, which checks for
the use of test methods outside of a test class.
Test methods should be defined within a test class to ensure their execution.

NOTE: This cop assumes that classes whose superclass name includes the word
"`Test`" are test classes, in order to prevent false positives.

```ruby
# bad
class FooTest < Minitest::Test
end
def test_method_should_be_inside_test_class
end

# good
class FooTest < Minitest::Test
  def test_method_should_be_inside_test_class
  end
end
```

config/default.yml

@@ -192,6 +192,12 @@ Minitest/NoTestCases:
   Enabled: false
   VersionAdded: '0.30'
 
+Minitest/NonExecutableTestMethod:
+  Description: 'Checks uses of test methods outside test class.'
+  Enabled: pending
+  Severity: warning
+  VersionAdded: '<<next>>'
+
 Minitest/NonPublicTestMethod:
   Description: 'Detects non `public` (marked as `private` or `protected`) test methods.'
   Enabled: pending

lib/rubocop/cop/minitest/non_executable_test_method.rb

@@ -0,0 +1,51 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Minitest
+      # Checks for the use of test methods outside of a test class.
+      #
+      # Test methods should be defined within a test class to ensure their execution.
+      #
+      # NOTE: This cop assumes that classes whose superclass name includes the word
+      # "`Test`" are test classes, in order to prevent false positives.
+      #
+      # @example
+      #
+      #   # bad
+      #   class FooTest < Minitest::Test
+      #   end
+      #   def test_method_should_be_inside_test_class
+      #   end
+      #
+      #   # good
+      #   class FooTest < Minitest::Test
+      #     def test_method_should_be_inside_test_class
+      #     end
+      #   end
+      #
+      class NonExecutableTestMethod < Base
+        include MinitestExplorationHelpers
+
+        MSG = 'Test method should be defined inside a test class to ensure execution.'
+
+        def on_def(node)
+          return if !test_method?(node) || !use_test_class?
+          return if node.left_siblings.none? { |sibling| possible_test_class?(sibling) }
+
+          add_offense(node)
+        end
+
+        def use_test_class?
+          root_node = processed_source.ast
+
+          root_node.each_descendant(:class).any? { |class_node| test_class?(class_node) }
+        end
+
+        def possible_test_class?(node)
+          node.is_a?(AST::ClassNode) && test_class?(node) && node.parent_class.source.include?('Test')
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/minitest_cops.rb

@@ -30,6 +30,7 @@
 require_relative 'minitest/assert_truthy'
 require_relative 'minitest/duplicate_test_run'
 require_relative 'minitest/empty_line_before_assertion_methods'
+require_relative 'minitest/non_executable_test_method'
 require_relative 'minitest/redundant_message_argument'
 require_relative 'minitest/return_in_test_method'
 require_relative 'minitest/test_file_name'

test/rubocop/cop/minitest/non_executable_test_method_test.rb

@@ -0,0 +1,158 @@
+# frozen_string_literal: true
+
+require_relative '../../../test_helper'
+
+class NonExecutableTestMethodTest < Minitest::Test
+  def test_registers_offense_when_test_method_is_defined_outside_minitest_test_class
+    assert_offense(<<~RUBY)
+      class FooTest < Minitest::Test
+      end
+      def test_foo
+      ^^^^^^^^^^^^ Test method should be defined inside a test class to ensure execution.
+      end
+    RUBY
+  end
+
+  def test_registers_offense_when_test_method_is_defined_outside_active_support_test_case_class
+    assert_offense(<<~RUBY)
+      class FooTest < ActiveSupport::TestCase
+      end
+
+      def test_foo
+      ^^^^^^^^^^^^ Test method should be defined inside a test class to ensure execution.
+      end
+    RUBY
+  end
+
+  def test_does_not_register_offense_when_test_method_is_defined_outside_namespaced_test_class
+    assert_offense(<<~RUBY)
+      module M
+        class FooTest < Minitest::Test
+          def test_foo
+          end
+        end
+
+        def test_bar
+        ^^^^^^^^^^^^ Test method should be defined inside a test class to ensure execution.
+        end
+      end
+    RUBY
+  end
+
+  def test_does_not_register_offense_when_non_test_method_is_defined_outside_test_class
+    assert_no_offenses(<<~RUBY)
+      class FooTest < Minitest::Test
+      end
+      def do_something
+      end
+    RUBY
+  end
+
+  def test_does_not_register_offense_when_non_test_method_is_defined_outside_active_support_test_case_class
+    assert_no_offenses(<<~RUBY)
+      class FooTest < ActiveSupport::TestCase
+      end
+      def do_something
+      end
+    RUBY
+  end
+
+  def test_does_not_register_offense_when_test_method_is_defined_inside_test_class
+    assert_no_offenses(<<~RUBY)
+      class FooTest < Minitest::Test
+        def test_foo
+        end
+      end
+    RUBY
+  end
+
+  def test_does_not_register_offense_when_test_method_is_defined_inside_test_helper_class
+    assert_no_offenses(<<~RUBY)
+      class FooTest < Minitest::Test
+        def test_foo
+        end
+      end
+
+      class TestHelperMailer < ActionMailer::Base
+        def test_parameter_args
+        end
+      end
+    RUBY
+  end
+
+  def test_does_not_register_offense_when_test_method_is_defined_inside_test_helper_module
+    assert_no_offenses(<<~RUBY)
+      class FooTest < Minitest::Test
+        def test_foo
+        end
+      end
+
+      module TestHelper
+        def test_parameter_args
+        end
+      end
+    RUBY
+  end
+
+  def test_does_not_register_offense_when_test_class_in_which_test_method_is_defined_is_repeated
+    assert_no_offenses(<<~RUBY)
+      class FooTest < Minitest::Test
+        def test_foo
+        end
+      end
+
+      class FooTest < Minitest::Test
+        def test_foo
+        end
+      end
+    RUBY
+  end
+
+  def test_does_not_register_offense_when_test_method_is_defined_and_test_class_is_not_defined
+    assert_no_offenses(<<~RUBY)
+      def test_something
+      end
+    RUBY
+  end
+
+  def test_does_not_register_offense_when_test_method_is_defined_inside_condition
+    assert_no_offenses(<<~RUBY)
+      module ActiveRecord
+        class AdapterTest < ActiveRecord::TestCase
+          unless current_adapter?(:PostgreSQLAdapter)
+            def test_update_prepared_statement
+            end
+          end
+        end
+      end
+    RUBY
+  end
+
+  def test_does_not_register_offense_when_nested_test_method_and_two_test_classes_are_defined
+    assert_no_offenses(<<~RUBY)
+      class FooTest < ActiveRecord::TestCase
+        def test_foo
+          def test_bar
+          end
+        end
+      end
+
+      class BarTest < ActiveRecord::TestCase
+      end
+    RUBY
+  end
+
+  def test_does_not_register_offense_when_non_test_case_class_is_defined_before_test_method
+    assert_no_offenses(<<~RUBY)
+      class SetTest < ActiveRecord::AbstractMysqlTestCase
+        class SetTest < ActiveRecord::Base
+        end
+
+        def test_should_not_be_unsigned
+          column = SetTest.columns_hash["set_column"]
+          assert_not_predicate column, :unsigned?
+        end
+      end
+    RUBY
+  end
+end