tests: Remove unneeded assert.h includes.

[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
http://testbot.winehq.org/JobDetails.pl?Key=14548

Your paranoid android.


=== WNT4WSSP6 (32 bit memallocator) ===
memallocator.c:34: Test failed: Unable to create memory allocator 80040154

=== WNT4WSSP6 (32 bit referenceclock) ===
referenceclock.c:97: Test failed: Unable to create reference clock from system clock 80040154