Increase Ready timeout for TestChannelConformance to 4 minutes (#414)

The default 2-minute timeout might not be enough in some scenarios where
deploying takes longer (like with Istio).

test/rekt/channel_test.go

@@ -48,6 +48,7 @@ func TestChannelConformance(t *testing.T) {
 		knative.WithTracingConfig,
 		k8s.WithEventListener,
 		environment.Managed(t),
+		environment.WithPollTimings(5*time.Second, 4*time.Minute),
 	)
 
 	channelName := "mychannelimpl"