Automatic merge from submit-queue

Client-gen: handle dotted group name, e.g., "authentication.k8s.io"

The client-gen used to assume the group name doesn't include dot, but it's not true, e.g., we have group `authentication.k8s.io`.

With this PR, Client-gen will use the full group name when creating directory (e.g., the client for the authentication group will be generated at pkg/client/clientset_generated/release_1_3/typed/`authentication.k8s.io`/v1/). However, because golang doesn't allow dot in variable/function names, so when the group name is used as part of variable/function name, client-gen extracts the part before the first dot (e.g., authentication).

This PR also changes the group name of the test group from `testgroup` to `testgroup.k8s.io` to verify if client-gen generates sane code.

cc @deads2k for #20573