pyocr: Use skip instead of expectedFailure

Since the update of imagemagick in
5e753c1a

 there are certain test cases
which now unexpectly succeed and in turn cause the whole build to fail.

So in order to prevent this from happening let's skip those tests
properly instead of running them and expect them to fail.

Tested by building pythonPackages.pyocr on x86_64-linux.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>