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.

Case in point:
http://permalink.gmane.org/gmane.comp.python.numeric.general/54182