Wait for the mount to complete before inspecting InitRequest/InitResponse.

This fixes flakiness in TestNoOpen.

Change-Id: I8909236d9539c1729bd8a78d460d6637f7c775e0

fuse/nodefs/fileless_test.go

@@ -60,6 +60,7 @@ func TestNoOpen(t *testing.T) {
 	defer s.Unmount()
 	go s.Serve()
 
+	s.WaitMount()
 	if s.KernelSettings().Minor < 23 {
 		t.Skip("Kernel does not support open-less read/writes. Skipping test.")
 	}