Previously, the fake RESTClient in client-go required a Registry. It
used the Registry to fetch the GroupVersion for the fake client.
However, the way it did so was dubious in some cases (it hard-coded the
default API group in places), and not strictly necssary.
This updates the fake client to just recieve the GroupVersion and
internal group name directly, instead of requiring a Registry, so that
it can be consumed in unit tests where a Registry isn't necessarily
readily available (e.g. elsewhere in client-go).
This is so that we can use NewAPIFactory() from cmd/set/*test.go
Up until now we would get a import loop error.
This commit also adds a basic unit test case for cmd/set/set_image.go
