Converge the pytest setups of runbot_merge and forwardport a bit
more (the goal is obviously to eventually share the infrastructure so
they run the same way).
Running multiple ngrok concurrently is only allowed from pro and up
(OOTB and without shenanigans) is only allowed from Pro and up. However
multiple tunnels through a single ngrok is allowed
-> when tunneling through ngrok, start the process without any tunnel,
use the API to create then remove the local tunnel, and shut down ngrok
IIF there's no tunnel left.
There's plenty of race conditions but given how slow tests are when they
involve github that's probably not an issue.
* extract method to create a PR object from a github result (from the
PR endpoint)
* move some of the remote's fixtures to a global conftest (so they can
be reused in the forwardbot)