user32/tests: Don't test function directly when reporting GetLastError()

[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=11234

Your paranoid android.


=== w7u (32 bit msg) ===
msg.c:12108: Test failed: unexpected hwnd 0014013C