tsnet: mark TestLoopbackLocalAPI as flakey

Test flaked in CI.

Updates  #8557

Signed-off-by: Charlotte Brandhorst-Satzkorn <charlotte@tailscale.com>

tsnet/tsnet_test.go

@@ -283,6 +283,7 @@ func TestConn(t *testing.T) {
 }
 
 func TestLoopbackLocalAPI(t *testing.T) {
+	flakytest.Mark(t, "https://github.com/tailscale/tailscale/issues/8557")
 	tstest.ResourceCheck(t)
 	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
 	defer cancel()