[test] Add verify-mockgen in CI

[gaocegege]

Now we do not check if generated code is changed, thus we should do it. Refer to https://github.com/kubeflow/tf-operator/blob/master/hack/verify-codegen.sh

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[andreyvelich]

/lifecycle frozen

[tenzen-y]

/assign