user32/tests: Ensure ShowWindow returns false when window is already hidden

[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=10433

Your paranoid android.


=== w7u (32 bit msg) ===
msg.c:12214: Test failed: unexpected hwnd 00080150
msg.c:12236: Test failed: bad time 30a5c/30a6b