winhttp/tests: Use a wait event instead of an unconditional sleep in the server thread.

[Marvin]

Hi,

While running your changed tests on Windows, I think I found new failures.
Being a bot and all I'm not very good at pattern recognition, so I might be
wrong, but could you please double-check?
Full results can be found at
https://testbot.winehq.org/JobDetails.pl?Key=11019

Your paranoid android.


=== w8 (32 bit winhttp) ===
winhttp.c:2042: Test failed: failed to receive response 12152
winhttp.c:2047: Test failed: request failed unexpectedly 327681