testtreelistmodel: Make the directory loading async

testtreelistmodel: Make the directory loading async

This is way more complicated than it should be, because it requires
manually limiting the number of open file enumerators.

On the other hand, it exhaustively tests the items-changed emission of
all involved listmodels because those signals come in pretty much
randomly.

It's also 50% slower than the sync version, with the caeat that the sync
version only shows the UI after it's done loading, while this version
shows it right away.