A tiny tweak to add the port chosen by stripe-mock to the "starting
stripe-mock" output. This gives the user a little more information
(which might be handy if something isn't working), and brings it inline
with Go's output format: https://github.com/stripe/stripe-go/pull/780
When starting a stripe-mock for a custom OpenAPI spec, pass `-http-port
0` on startup, which tells stripe-mock to select a port, then extract
that port from its output.
This is not a total win because we now have to rely on string matching,
but it is better in that (1) it gets a port more efficiently, (2) it
eliminates a race condition where another process could take the port we
found before stripe-mock gets to start, and (3) it starts a little
faster as we take advantage of the fact that we know stripe-mock has
started when we've found a port in its output (in my tests it took ~0.2
to 0.3 seconds compared to a 1 second sleep).
