[Reviewer: Andy] Interop enhancements

Some changes to the live tests coming out of interop testing. Specifically:

* Better handling of provisional responses, including barriers to stop some provisional responses being absorbed when a subsequent final response arrives too quickly, and an environment variable to allow other provisional responses to be absorbed
* Fixes to our SUBSCRIBE tests, which attempted to create an invalid dialog and didn't catch a CSeq bug
* A couple of monkey-patched utility methods to send INVITEs/200s with valid but ineffective SDP, and to receive a 200 and a NOTIFY (in any order) and return the NOTIFY
* More permissive checking of P-Associated-URI headers (to support P-CSCFs that convert them into a non-spec-compliant form)
* The ability to exclude tests, for cases where bugs in interop test kit mean that certain testcases just doesn't work in certain environments

This picks up Quaff 0.7.0.
Bit nasty, especially the way we end up using it.  Would the discussed `maybe_recv` be a better solution?