[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/exec_test.go

@@ -131,7 +131,7 @@ func TestPodAndContainer(t *testing.T) {
 	for _, test := range tests {
 		f, tf, _, 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) { return nil, nil }),
 		}
@@ -186,7 +186,7 @@ func TestExec(t *testing.T) {
 	for _, test := range tests {
 		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) {
 				switch p, m := req.URL.Path, req.Method; {