BUG: testing: always enable --exe

Setuptools tends to set +x to the installed test scripts, which makes
numpy.test() to not run any tests. Having --exe always enabled is not
problematic because only files matching 'test_*.py' are looked into.

For instance, if you installed Numpy with `setupegg.py`, you cannot run 
any tests. This issue really should be fixed.

Cases in point:

http://comments.gmane.org/gmane.comp.python.numeric.general/32309
http://mail.scipy.org/pipermail/scipy-dev/2012-May/017591.html
http://permalink.gmane.org/gmane.comp.python.numeric.general/54182

It seems that there has been some discussion about this before, but I don't
believe it is a correct decision to just say "we don't support setuptools, don't
use it" while offering it as a build method.