Update/reformat maintainer entries that I am a part of
- Replace all instances of freebsd-testing with #test. #test is the Phabricator group that focuses on test-related reviews.
- Replace atf with contrib/atf, as that's the actual location for the test framework.
- Remove jmmv@ from the maintainers list for atf. He is the upstream project owner, but was moved to alumni status after r345787.
- Fix a typo accidentally introduced in r346899 (inpact -> impact).