winhttp/tests: Constify a character string

[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://newtestbot.winehq.org/JobDetails.pl?Key=3905

Your paranoid android.


=== w7pro64 (32 bit winhttp) ===
winhttp.c:859: Test failed: failed to send request 12175