TST: Fix ImportError in runtests.py (fixes #5273)

I think the problem is because numpy.testing has only been partially imported when it tries to import numpy.utils, so when numpy.core tries `from numpy.testing import Tester` it can't find it. I've fixed this by moving the import of Tester to before `from .utils import *`. However I'm not sure why this problem hasn't appeared for others -- perhaps I am doing something else wrong.