Rather than add individual tunnel methods to conftest, just allows
specifying a tunnel script and have that do whatever.
Protocol is uncomplicated: workers run the `$tunnel` with an arbitrary
port, script should create a tunnel to `localhost:$port`, print the
ingress of the tunnel to `STDOUT` with a terminating newline then
close `STDOUT`, and wait for `SIGINT` or `SIGTERM`, doing whatever
cleanup they need when receiving either of those.
`ngrok` and `localtunnel` adapter scripts are provided out of the box,
though the ngrok one doesn't *really* work when using xdist without a
pre-spawned ngrok worker. Then again using xdist against github actual
is suicidal (because concurrency limits + rate limits) so likely
irrelevant.
Fixes#729
