Some DNS NXDOMAIN hijackers hijack truly ridiculous domains, like
"invalid-stuff!!" or "1.2.3.4.5". This would provoke unit test
failures where we used addresses like that to force
tor_addr_lookup() to fail. The fix, for testing, is to mock
tor_addr_lookup() with a variant that always fails when it gets
a name with a !.
Fixes bugs 20862 and 20863.
