[Reviewer: Alex] Prevent pauses when running tests without an accessible DNS server

Turns out a five-hour plane journey spent working on FakeLogger really concentrates the mind on https://github.com/Metaswitch/sprout/issues/588. (I think DNS caching might have been misleading me before.)

The problem was just that some of the trusted peer processing doesn't go through DnsResolver but goes through pj_sockaddr_parse instead, so SipTest::add_host_mapping wasn't actually mocking the DNS lookup out. I'll talk to Mike about whether using pj_sockaddr_parse is right, but for now, I've added cwtest_add_host_mapping (back?) in to avoid this and potential future issues.

I do still see a brief pause after some of the emergency registration tests - the logs suggest this is because SipResolver is doing some work - but the IBCF-related pauses were far longer and more annoying.