Automatic merge from submit-queue (batch tested with PRs 53204, 53364, 53559, 53589, 53088). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.
pkg/api: move *_test.go -> pkg/api/testing
Cleaning up pkg/api, one little step towards pkg/apis/core.