* wrap/python/tests/setxor.py, wrap/python/tests/bddnqueen.py:
Clean all used bdd variables before calling bdd_done(), so that
bdd_delref() is never called after bdd_done(). In NDEBUG builds,
bdd_delref() does not check whether the BuDDy is running or not,
and calling it after bdd_done() will crash.