Add a test for timeout'ing HTLCs which claim to be a part of an MPP

Add a test for timeout'ing HTLCs which claim to be a part of an MPP

This is a key test for our automatic HTLC time-out logic, as it
ensures we don't allow an HTLC which indicates we should wait for
additional HTLCs before responding to cause us to force-close a
channel due to HTLC near-timeout.