Automatic merge from submit-queue (batch tested with PRs 41921, 41695, 42139, 42090, 41949)
Unify fake runtime helper in kuberuntime, rkt and dockertools.
Addresses https://github.com/kubernetes/kubernetes/pull/42081#issuecomment-282429775.
Add `pkg/kubelet/container/testing/fake_runtime_helper.go`, and change `kuberuntime`, `rkt` and `dockertools` to use it.
@yujuhong This is a small unit test refactoring PR. Could you help me review it?