Before this change, the way to add messages had a lot
of ping-pong-ing between MessageList and MessageListData,
where first the data got triaged, but not actually
inserted into data structures, and then subsequent
calls would add the data and get filtered results.
Now we have a simple API for MessageListData.add_messages
that does all the data stuff up front. Having a fully
function MLD.add_messages not only makes the ML.add_messages
function about four lines shorter, it also sets us up
to easily build standalone MLD objects before making
the heavier ML objects.
This run_test helper sets up a convention that allows
us to give really short tracebacks for errors, and
eventually we can have more control over running
individual tests. (The latter goal has some
complications, since we often intentionally leak
setup in tests.)
