Enable Minitest/NonExecutableTestMethod cop

This pull request enables `Minitest/NonExecutableTestMethod` cop
to find non-executed test that is out of `ActiveSupport::TestCase` and its subclasses.

This cop is based on the request since there was a test that is not executed found
at rails#50334 (comment)
and implemented to RuboCop Minitest 0.34.0 via rubocop/rubocop-minitest#279

This cop works as follows.
As of right now, there is no offenses by reverting the merge commit via rails#50334

```
$ git revert -m 1 9517841
$ bundle exec rubocop
Inspecting 3254 files
... snip ...

Offenses:

activerecord/test/cases/assertions/query_assertions_test.rb:27:5: W: Minitest/NonExecutableTestMethod: Test method should be defined inside a test class to ensure execution.
    def test_assert_no_queries ...
    ^^^^^^^^^^^^^^^^^^^^^^^^^^

3254 files inspected, 1 offense detected
$
```

* `Gemfile.lock` has been updated as follows
```
bundle update rubocop rubocop-minitest --conservative
```

.rubocop.yml

@@ -367,6 +367,9 @@ Minitest/AssertWithExpectedArgument:
 Minitest/LiteralAsActualArgument:
   Enabled: true
 
+Minitest/NonExecutableTestMethod:
+  Enabled: true
+
 Minitest/SkipEnsure:
   Enabled: true