Automatic merge from submit-queue (batch tested with PRs 64911, 65710). 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>.
apiextensions-apiserver: add pkg/cmd/server/testing pkg for integration tests
In analogy to kube-apiserver, this implements a uniform apiextensions-apiserver for integration tests which can be started using customized flags.
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