[client-go] avoid Registry in fake REST client

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).

pkg/kubectl/cmd/set/set_selector_test.go

@@ -318,7 +318,7 @@ func TestGetResourcesAndSelector(t *testing.T) {
 func TestSelectorTest(t *testing.T) {
 	f, tf, codec, ns := cmdtesting.NewAPIFactory()
 	tf.Client = &fake.RESTClient{
-		APIRegistry:          legacyscheme.Registry,
+		GroupVersion:         legacyscheme.Registry.GroupOrDie(api.GroupName).GroupVersion,
 		NegotiatedSerializer: ns,
 		Client: fake.CreateHTTPClient(func(req *http.Request) (*http.Response, error) {
 			t.Fatalf("unexpected request: %s %#v\n%#v", req.Method, req.URL, req)