winmm/tests: Add a small Sleep() to prevent a notification test from failing.

[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=25773

Your paranoid android.


=== w864 (32 bit mci) ===
mci: unhandled exception c0000005 at 77479E80

=== w1064 (32 bit mci) ===
mci: unhandled exception c0000005 at 7698851A