In particular, tests showing that #3824 works. But that test doesn't
actually work yet; it only gets a DERP connection. (why?)
Updates #13038
Change-Id: Ie1fd1b6a38d4e90fae7e72a0b9a142a95f0b2e8f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>