tstest/integration: mark TestTwoNodes as flaky

I couldn't reproduce this locally, but it's failing pretty consistently
for me in CI.

Updates #11960

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Id78b063a3b8768e2dfe47b5ab1c0cab36dcb53eb

tstest/integration/integration_test.go

@@ -345,6 +345,7 @@ func TestConfigFileAuthKey(t *testing.T) {
 
 func TestTwoNodes(t *testing.T) {
 	tstest.Shard(t)
+	flakytest.Mark(t, "https://github.com/tailscale/tailscale/issues/11960")
 	tstest.Parallel(t)
 	env := newTestEnv(t)