net/porttrack: add net.Listen wrapper to help tests allocate ports race-free

Updates tailscale/corp#27805
Updates tailscale/corp#27806
Updates tailscale/corp#37964

Change-Id: I7bb5ed7f258e840a8208e5d725c7b2f126d7ef96
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f