user32/tests: Avoid side-effect in an assert.

[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=12631

Your paranoid android.


=== wvistau64 (32 bit win) ===
win.c:7487: Test failed: didn't get start_event
in.c:7429: Test failed: button under static window didn't get WM_LBUTTONUP