Added initial tests for repository._transform.

Also added comments re. failed tests for repository._transform. This is for #384

Thank you for your contribution, @flowejam!

The tests are failing! Could you please fix that?

Thanks @suhaibmujahid !

Apologies if I'm missing something here, but from the log it looks like the only test that is failing is the one I added - I've included a snippet below. I think there could be an issue with the way tests are detected?

  # The test currently fails here with an error - might indicate an issue with
        # the is_test function in bugbug/repository.py.
        repository.transform(hg, local, commit3)
>       assert commit3.test_files_modified_num == 1
E       AssertionError: assert 0 == 1

E.g. for bugbug, tests are in a folder called 'tests' in the main directory of the repo, but wouldn't they be missed by this function (repository.is_test)?:

# This code was adapted from https://github.com/mozsearch/mozsearch/blob/2e24a308bf66b4c149683bfeb4ceeea3b250009a/router/router.py#L127
def is_test(path: str) -> bool:
    return (
        "/test/" in path
        or "/tests/" in path
        or "/mochitest/" in path
        or "/unit/" in path
        or "/gtest/" in path
        or "testing/" in path
        or "/jsapi-tests/" in path
        or "/reftests/" in path
        or "/reftest/" in path
        or "/crashtests/" in path
        or "/crashtest/" in path
        or "/gtests/" in path
        or "/googletest/" in path
    )

@suhaibmujahid @marco-c just thought I'd follow up - any chance of getting another look at this?