ntdll/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
http://testbot.winehq.org/JobDetails.pl?Key=7877

Your paranoid android.


=== W7PROX64 (32 bit exception) ===
No test summary line found

=== WVISTAADM (32 bit directory) ===
TestLauncher.c:89: Test failed: Can't open test executable C:\winetest\ntdll_test.exe, error 32