[client-go] fake discovery returns server groups

The fake discovery client currently returns `nil, nil` for several
methods.  Among them is the `ServerGroups` method, which is used by the
discovery REST mapper implementations.  This updates the fake discovery
client to actually return server groups so that the discovery REST
mapper can be used in tests.

staging/src/k8s.io/client-go/discovery/fake/discovery.go

@@ -68,7 +68,44 @@ func (c *FakeDiscovery) ServerPreferredNamespacedResources() ([]*metav1.APIResou
 }
 
 func (c *FakeDiscovery) ServerGroups() (*metav1.APIGroupList, error) {
-	return nil, nil
+	action := testing.ActionImpl{
+		Verb:     "get",
+		Resource: schema.GroupVersionResource{Resource: "group"},
+	}
+	c.Invokes(action, nil)
+
+	groups := map[string]*metav1.APIGroup{}
+
+	for _, res := range c.Resources {
+		gv, err := schema.ParseGroupVersion(res.GroupVersion)
+		if err != nil {
+			return nil, err
+		}
+		group := groups[gv.Group]
+		if group == nil {
+			group = &metav1.APIGroup{
+				Name: gv.Group,
+				PreferredVersion: metav1.GroupVersionForDiscovery{
+					GroupVersion: res.GroupVersion,
+					Version:      gv.Version,
+				},
+			}
+			groups[gv.Group] = group
+		}
+
+		group.Versions = append(group.Versions, metav1.GroupVersionForDiscovery{
+			GroupVersion: res.GroupVersion,
+			Version:      gv.Version,
+		})
+	}
+
+	list := &metav1.APIGroupList{}
+	for _, apiGroup := range groups {
+		list.Groups = append(list.Groups, *apiGroup)
+	}
+
+	return list, nil
+
 }
 
 func (c *FakeDiscovery) ServerVersion() (*version.Info, error) {