Add a test for mypy result regex matching

[brettcannon]

A test for bfcd225 .

Need to also check that the regex doesn't affect what mypy checks and is only for teasing out results (e.g. this won't cause mypy to check a README file suddenly).

[ericsnowcurrently]

Why is that check (target vs. results) needed relative to MyPy? From what I can tell, the handled filenames are determined elsewhere in the extension.

[DonJayamanne]

rom what I can tell, the handled filenames are determined elsewhere in the extension.

That's because the file name is not part of the result. We run linters on a file by file basis, hence at the calling location we know the file that is being linted.