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.

codersdk/workspacesdk/agentconn_test.go

@@ -243,7 +243,9 @@ func stitchTailnet(t *testing.T, conns map[uuid.UUID]*tailnet.Conn) {
 				Node: protoNode,
 				Kind: proto.CoordinateResponse_PeerUpdate_NODE,
 			}})
-			assert.NoError(t, err)
+			if err != nil && !errors.Is(err, tailnet.ErrConnClosed) {
+				assert.NoError(t, err)
+			}
 		}
 	}