Update new format_checkers tests to use pathlib

[terriko]

I had a bit of a race condition with two PRS:

• refactor: use pathlib.Path instead of os.path #1714 updated our tests to use pathlib

• At the same time, test: added unit tests for format_checkers script #1709 was adding new format_checkers tests that used the older os.path.

I'm opening this issue as a reminder that we should refactor the new tests in #1709 to also use pathlib since I'm expecting to merge #1709 when the tests are done running.