negative ints *as* negative ints when fed into %x formats.  Python 2.3
still renders them as positive ints, but spews

FutureWarning: %u/%o/%x/%X of negative int will return a signed string
               in Python 2.4 and up

to warn about the impending change.  Jim reported two instances of that
warning when running the tests on a box where addresses happen to "be
negative".  So make the addresses look positive instead (2.3 and 2.4
treat those the same, so 2.3 doesn't warn about those).

Problem:  it occurs to me now that I'm assuming addresses fit in 32
bits here.