We're seeing some flaky tests related to agent connectivity - https://github.com/coder/coder/actions/runs/7286675441/job/19856270998

I'm pretty sure what happened in this one is that the client opened a connection while the wgengine was in the process of reconfiguring the wireguard device, so the fact that the peer became "active" as a result of traffic being sent was not noticed.

The test calls `AwaitReachable()` but this only tests the disco layer, so it doesn't wait for wireguard to come up.

I think we should be using TSMP for pinging and reachability, since this operates at the IP layer, and therefore requires that wireguard comes up before being successful.

This should also help with the problems we have seen where a TCP connection starts before wireguard is up and the initial round trip has to wait for the 5 second wireguard handshake retry.

fixes: #11294