use synctest for the packet drop test (#5393)

integrationtests/self/simnet_helper_test.go

@@ -0,0 +1,18 @@
+package self_test
+
+import "github.com/quic-go/quic-go/testutils/simnet"
+
+type droppingRouter struct {
+	simnet.PerfectRouter
+
+	Drop func(simnet.Packet) bool
+}
+
+func (d *droppingRouter) SendPacket(p simnet.Packet) error {
+	if d.Drop(p) {
+		return nil
+	}
+	return d.PerfectRouter.SendPacket(p)
+}
+
+var _ simnet.Router = &droppingRouter{}