Improve test_helper files method

[Mth0158]

The files instance method should micmic the fixture_path instance method behaviour (ie linking to class method see https://github.com/rails/marcel/blob/170458c687ed22f07d8829043a04e008a2b1936b/test/test_helper.rb#L52).

This could avoid future issues with 2 files methods supposedly identical not giving the same output.