Reserve HTTP server port

[pNre]

Ciao,
we've been relying on SBTUITestTunnel to UI tests a fairly large codebase with 2k+ UI tests.
When running tests in parallel on multiple simulators (4+) with IPC disabled, we've been encountering failures due to UITestTunnelServer being unable to bind to the HTTP server address:

[UITestTunnelServer] Failed to start server on port 61692. Error Domain=NSPOSIXErrorDomain Code=48 "Address already in use" UserInfo={NSLocalizedDescription=Address already in use}

Through debugging I've identified a potential race in [SBTUITestTunnelClient findOpenPort] which can lead to the system allocating the same port in parallel processes, leading to the aforementioned error.

In this PR I've refactored findOpenPort to temporarily "reserve" the port which should prevent the system from allocating it to other processes for a certain time frame (hopefully long enough to start the server and bind the address)

[tcamin]

Ciao! Could you elaborate, possibly expanding the code comment you added, to explain why this is actually working? I may be wrong but it seems that the trick of "reserving" the port by opening/closing a socket temporarily works because SO_REUSEADDR is set here and the system won't reuse the port while there are connection in TIME_WAIT state. If this is the case a more in-depth explanation would greatly help to understand that part of the code.

Thanks!

[pNre]

Yes, that's the idea. I've amended the comment to provide a more detailed explanation.

[tcamin]

Great fix, thanks!