prune sig-testing teams
prune sig-testing teams

These were individual commits, but probably not needed, so:

- Drop test-infra-reviewers in favor of sig-testing-pr-reviews
- Demote all sig-testing team maintainers to members
- ... except those who are GitHub Owners because GitHub, apparently
- Add akutz
- Fix Katharine's case
- Update team descriptions
- test-infra-admins ~= kubernetes/test-infra/OWNERS
- test-infra-maintainers ~= kubernetes/test-infra/OWNERS
- sig-testing-pr-reviews contains test-infra-admins

kubernetes/org,kubernetes/org,kubernetes/org