user32/tests: Flush pending events before testing menu 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://newtestbot.winehq.org/JobDetails.pl?Key=4981

Your paranoid android.


=== w8 (32 bit msg) ===
msg.c:5965: Test failed: Regions are different
msg.c:5965: Test failed: Regions are different
msg.c:5965: Test failed: Regions are different
msg.c:8366: Test failed: did not get expected count for minimum timeout (54 != ~64).
msg.c:12089: Test failed: unexpected message 31f
msg.c:12090: Test failed: bad wparam 1
msg.c:12100: Test failed: unicode WM_CHAR: 0: the msg sequence is not complete: expected 0102 - actual 0000