Fix TestValidRemoteDriver GetCapabilities errors

This test was producing error messages due to missing endpoints
in the plugin API;

```
=== RUN   TestValidRemoteDriver
ERRO[0039] error getting capability for valid-network-driver due to NetworkDriver.GetCapabilities: 404 page not found
 ```

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

libnetwork/libnetwork_test.go

@@ -1375,6 +1375,10 @@ func TestValidRemoteDriver(t *testing.T) {
 		w.Header().Set("Content-Type", "application/vnd.docker.plugins.v1+json")
 		fmt.Fprintf(w, `{"Implements": ["%s"]}`, driverapi.NetworkPluginEndpointType)
 	})
+	mux.HandleFunc(fmt.Sprintf("/%s.GetCapabilities", driverapi.NetworkPluginEndpointType), func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/vnd.docker.plugins.v1+json")
+		fmt.Fprintf(w, `{"Scope":"local"}`)
+	})
 	mux.HandleFunc(fmt.Sprintf("/%s.CreateNetwork", driverapi.NetworkPluginEndpointType), func(w http.ResponseWriter, r *http.Request) {
 		w.Header().Set("Content-Type", "application/vnd.docker.plugins.v1+json")
 		fmt.Fprintf(w, "null")