winhttp/tests: Add an authentication test using IWinHttpRequest.

[Hans Leidekker]

It doesn't build cleanly for me:

winhttp.c: In function ‘test_IWinHttpRequest’:
winhttp.c:3631:5: warning: implicit declaration of function ‘sprintf’ [-Wimplicit-function-declaration]
     sprintf( buf, "http://localhost:%d/auth", port );
     ^
winhttp.c:3631:5: warning: incompatible implicit declaration of built-in function ‘sprintf’