testsuite: Add a treelistmodel test

testsuite: Add a treelistmodel test

This one exhaustively tests reusing the same model as a child model for
many nodes.
This tracks that multiple items-changed signals emitted at the same time
(or multiple handlers for one such signal) doesn't put the treelistmodel
in an inconsistent state while it is handling all of them.

I'm not sure this (ab)use of treelistmodel should be officially
supported, but it works today, so let's test it to see if we can keep it
working.