[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