[Fix rubocop#216] Add new Minitest/NoTestCases cop

Closes rubocop#216

changelog/new_no_test_cases_cop.md

@@ -0,0 +1 @@
+* [#216](https://github.com/rubocop/rubocop-minitest/issues/216): Add new `Minitest/NoTestCases` cop. ([@tejasbubane][])

config/default.yml

@@ -164,6 +164,11 @@ Minitest/NoAssertions:
   Enabled: false
   VersionAdded: '0.12'
 
+Minitest/NoTestCases:
+  Description: 'Checks if test class contains any test cases.'
+  Enabled: pending
+  VersionAdded: '<<next>>'
+
 Minitest/RefuteEmpty:
   Description: 'This cop enforces to use `refute_empty` instead of using `refute(object.empty?)`.'
   StyleGuide: 'https://minitest.rubystyle.guide#refute-empty'

lib/rubocop/cop/minitest/no_test_cases.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Minitest
+      # Checks if test class contains any test cases.
+      #
+      # @example
+      #   # bad
+      #   class FooTest < Minitest::Test
+      #     def do_something
+      #     end
+      #   end
+      #
+      #   # good
+      #   class FooTest < Minitest::Test
+      #     def test_something
+      #       assert true
+      #     end
+      #   end
+      #
+      class NoTestCases < Base
+        include MinitestExplorationHelpers
+
+        MSG = 'Test class has no test cases.'
+
+        def on_class(node)
+          return unless test_class?(node)
+
+          add_offense(node) if test_cases(node).empty?
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/minitest_cops.rb

@@ -31,6 +31,7 @@
 require_relative 'minitest/literal_as_actual_argument'
 require_relative 'minitest/multiple_assertions'
 require_relative 'minitest/no_assertions'
+require_relative 'minitest/no_test_cases'
 require_relative 'minitest/refute_empty'
 require_relative 'minitest/refute_false'
 require_relative 'minitest/refute_equal'

test/rubocop/cop/minitest/no_test_cases_test.rb

@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+
+class NoTestCases < Minitest::Test
+  def test_registers_offense_for_empty_test_class
+    assert_offense(<<~RUBY)
+      class FooTest < Minitest::Test
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Test class has no test cases.
+      end
+    RUBY
+  end
+
+  def test_registers_offense_for_nonempty_test_class_with_no_test_cases
+    assert_offense(<<~RUBY)
+      class FooTest < Minitest::Test
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Test class has no test cases.
+        def perform
+          1 + 2
+        end
+      end
+    RUBY
+  end
+
+  def test_does_not_register_offense_when_test_class_has_test_case
+    assert_no_offenses(<<~RUBY)
+      class FooTest < Minitest::Test
+        def test_foo; end
+      end
+    RUBY
+  end
+
+  def test_does_not_register_offense_for_non_test_classes
+    assert_no_offenses(<<~RUBY)
+      class FooTest
+        def perform; end
+      end
+    RUBY
+  end
+end