[Reviewer: Andy] Accept-Contact test suite

Builds on the refactoring reviewed in https://github.com/Metaswitch/clearwater-live-test/pull/64. This makes a few more changes (a common Endpoint base class for SIPpEndpoint and QuaffEndpoint, as a sensible place to do the instance_id calculations needed to make sure multiple bindings don't overwrite each other) and adds five tests of contact filtering function.
Is this https://github.com/RobertDober/Forwarder? If so we should add it to the `Gemfile`.