New cop to detect returns in test cases

[fatkodima]

# bad
def test_check_constraints
  return if mysql?
  # do testing
end

# good
def test_check_constraints
  skip("MySQL does not support check constraints") if mysql?
  # do testing
end

Example: https://github.com/gregnavis/active_record_doctor/blob/fc736812db046e2c0bfdbb6cec0b837e01a35076/test/active_record_doctor/detectors/mismatched_foreign_key_type_test.rb#L6

When using skip, minitest will report a skipped tests summary at the end; --verbose and --show-skips will show skips (https://github.com/minitest/minitest/blob/0984e29995a5c0f4dcf3c185442bcb4f493ed5e3/lib/minitest.rb#L208-L214); minitest-reporters will show the skipped tests; skip requires a reason with the existing cop (SkipWithoutReason) and overall it is more clear the intention, than just simple return.

[koic]

This seems good idea. Can you open to the Minitest Style Guide first?
https://github.com/rubocop/minitest-style-guide

[fatkodima]

@tejasbubane Thank you for the PR into the style guide! Do you want to implement this check too?

[tejasbubane]

@fatkodima Yes I'll work on the implementation within a day or two.