make test_set_verify_callback_exception pass on the *BSD

There is a timing problem in the code that tests the verification callback that apparently occurs in BSD systems (at least FreeBSD and NetBSD, see https://github.com/pyca/pyopenssl/issues/38). This small fix seems to do the trick for me, although I can't remember exactly why (it was a month ago that I tested it). Hope @koobs and @0-wiz-0 can confirm that it works for them too.