winmm/tests: Change the current directory to a place that will be writable.

[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=5140

Your paranoid android.


=== w8 (32 bit mci) ===
mci.c:1123: Test failed: got 0001 instead of MCI_NOTIFY_xyz 0004 from command play (aborted by close)