user32/tests: Add a test for sending inter-thread messages.

[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=12880

Your paranoid android.


=== w7u (32 bit msg) ===
msg.c:6957: Test failed: destroy child on thread exit: 0: the msg 0x0082 was expected, but got msg 0x0014 instead
msg.c:6957: Test failed: destroy child on thread exit: 1: the msg 0x000f was expected, but got msg 0x0082 instead
msg.c:6957: Test failed: destroy child on thread exit: 2: the msg 0x0014 was expected, but got msg 0x000f instead
msg.c:6957: Test failed: destroy child on thread exit: 3: the msg sequence is not complete: expected 0000 - actual 0014
msg.c:12230: Test failed: unexpected hwnd 00060150
msg.c:12252: Test failed: bad time 31cc3/31cd2