Update/reformat maintainer entries that I am a part of

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).