Add test cases for problem matcher logic

[jamescurtin]

    > This looks great!

Thanks.

Do you think it would be possible to add a job to the test workflow that asserts the regex in isort-matcher.json matches against an expected file?

Could we compromise on merging this as is and creating an issue in the project assigned to me, then I'll try and do a more general helper action to test problem matchers, as the point you raise is valid for the others I use elsewhere too?

If isort's error message ever changed, this should be sufficient to let us know an additional pattern would need to be added.

If it was ever going to be changed, it would be beneficial to encourage them to add a separator between the filename and the error message, especially if they started to add additional error messages in future...

Originally posted by @peternewman in https://github.com/isort/isort-action/issues/66#issuecomment-1339510399