improve testing of route failover logic

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

integration/route_test.go

@@ -252,7 +252,7 @@ func TestHASubnetRouterFailover(t *testing.T) {
 
 	scenario, err := NewScenario()
 	assertNoErrf(t, "failed to create scenario: %s", err)
-	defer scenario.Shutdown()
+	// defer scenario.Shutdown()
 
 	spec := map[string]int{
 		user: 3,