Add test annotation for test tuning

* Thanks to pytest mark, we can select tests with -m module (to execute
* only module annotate class) and -m not module (to execute only not
* annotate class)
On Mon, Aug 18, 2014 at 01:56:10PM -0700, pbrunet wrote:
> In pythran/tests/test_env.py:
> > @@ -12,6 +12,7 @@
> >  from numpy import ndarray
> >  import numpy.testing as npt
> >  import ast
> > +import pytest
> I can do it. I just wonder if requiring pytest is not a good solution to avoid
> this extra code? (In this case, the setup.py could be cleaned up)

OK, let's cleanup the code.