add simnet package to simulate a net.PacketConn in memory (#5385)

* Implement simnet

* simnet: remove separate license

* simnet: remove go.mod, use standard require package

* simnet: add README

* simnet: use synctest wrapper in tests

* simnet: minor code cleanup

* simnet: expose Packet.Data

* simnet: explose Simnet.Router

* simnet: remove SimpleFirewallRouter

* simnet: remove stray fmt.Println in tests

* fix deadline check for write deadlines

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* simnet: fix SetReadDeadline logic

---------

Co-authored-by: Marco Munizaga <git@marcopolo.io>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

testutils/simnet/README.md

@@ -0,0 +1,16 @@
+# simnet
+
+This package is based on @MarcoPolo's [simnet](https://github.com/marcopolo/simnet) package.
+
+A small Go library for simulating packet networks in-process. It provides
+drop-in `net.PacketConn` endpoints connected through configurable virtual links
+with bandwidth, latency, and MTU constraints. Useful for testing networking code
+without sockets or root privileges.
+
+- **Drop-in API**: implements `net.PacketConn`
+- **Realistic links**: per-direction bandwidth, latency, and MTU
+- **Backpressure/buffering**: bandwidth–delay product aware queues
+- **Routers**: perfect delivery, fixed-latency, simple firewall/NAT-like routing
+- **Deterministic testing**: opt-in `synctest`-based tests for time control
+
+