[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.
As discussed, RFC 3261 requires that a stateful proxy forward all provisional responses:

```
         Until a final response has been sent on the server transaction,
         the following responses MUST be forwarded immediately:

         -  Any provisional response other than 100 (Trying)
```

Given this, you're happy that we require an explicit flag if we don't expect this behaviour, rather than implicitly accepting it (and possibly blinding ourselves to future Clearwater bugs).