tstest/integration: mark TestNATPing flaky

Updates #12169

Signed-off-by: Maisem Ali <maisem@tailscale.com>

tstest/integration/integration_test.go

@@ -842,6 +842,7 @@ func TestClientSideJailing(t *testing.T) {
 // TestNATPing creates two nodes, n1 and n2, sets up masquerades for both and
 // tries to do bi-directional pings between them.
 func TestNATPing(t *testing.T) {
+	flakytest.Mark(t, "https://github.com/tailscale/tailscale/issues/12169")
 	tstest.Shard(t)
 	tstest.Parallel(t)
 	for _, v6 := range []bool{false, true} {