python/pillow: Only disable failing tests

There's no reason to disable ALL tests just because only one particular
test module is failing.

Tested on i686-linux and x86_64-linux against these Python versions:

  Python 2.6: The interpreter itself doesn't build
  Python 2.7: Successful for both architectures
  Python 3.3: Successful for both architectures
  Python 3.4: Successful for both architectures
  Python 3.5: Successful for both architectures
  Python 3.6: One of the dependencies of pillow doesn't build (pytest)

Tests for PyPy still fail, which is why the doCheck attribute is only
set to false if we're building for PyPy.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @desiderius, @goibhniu, @prikhi