[6/12] ntdll/tests: Add tests for threadpool timer merging.

[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=14965

Your paranoid android.


=== w7u (32 bit threadpool) ===
threadpool.c:856: Test failed: WaitForSingleObject returned 258
threadpool.c:897: Test failed: expected that ticks are nonzero
threadpool.c:899: Test failed: expected that timers are merged