Fix failing PlaceTreeModelTest autotest

Since Frameworks 5.41, `PlaceTreeModelTest` would fail. Bisecting shows
this has been caused by 7eb6333bdb48 in KIO ("Added baloo urls into
places model"), where the model now returns 4 additional entries for
"Recently Saved" and another 4 entries for "Search For".

While the fix is trivially done by changing the number of expected
items, in a way the test functioned as a canary for problems deeper in
the code. In particular it uncovered problems when accessing the newly
added virtual folders as well as various usability issues. Some of those
are fixed already (50e6fa3f and 86d75454), but there is still
some work left to do.

BUG: 387824

Test Plan: `placetreemodeltest` does not fail with KF 5.41 anymore,
still works with KF 5.40.