setup.py: add option to run tests under valgrind
setup.py: add option to run tests under valgrind

setup.py test --valgrind

This uses the system provided suppression files.
We might want to move this into runtests.py later.

pexip/pygobject,GNOME/pygobject,GNOME/pygobject,pexip/pygobject,pexip/pygobject,GNOME/pygobject