usp10/test: make sure to only trace on _test_shape_ok number of glyphs if font is not valid

[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=11158

Your paranoid android.


=== WNT4WSSP6 (32 bit usp10) ===
Timeout