Closes https://github.com/coder/internal/issues/1620
Closes ENG-3043
The callback cleanup added in #20687 stops new node callbacks, but it
can still race with one that's already in flight. That callback may call
`UpdatePeers` after the destination `tailnet.Conn` closes, so the test
fails with `connection closed` even though the redirect behaviour is
correct.
To fix, we'll just ignore `tailnet.ErrConnClosed` in the asynchronous
`stitch` helpers, whilst continuing to assert on every other error.
Ethan
4dbb3a236c