test: fix tailnet connection teardown flake (#27768)

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.

tailnet/conn_test.go

@@ -2,6 +2,7 @@ package tailnet_test
 
 import (
 	"context"
+	"errors"
 	"net"
 	"net/netip"
 	"strings"
@@ -483,7 +484,9 @@ func stitch(t *testing.T, dst, src *tailnet.Conn) {
 			Node: pn,
 			Kind: proto.CoordinateResponse_PeerUpdate_NODE,
 		}})
-		assert.NoError(t, err)
+		if err != nil && !errors.Is(err, tailnet.ErrConnClosed) {
+			assert.NoError(t, err)
+		}
 	})
 	// ensures we don't send callbacks after the test ends and connections are closed.
 	t.Cleanup(func() {