When creating multiple nested new folders, one at a time, in the "save as"
dialog, where folders are created and entered, a dirlister would hit an
assert (in DirItem::reinsert()), because one of the created folders would
eventually get inserted in pendingUpdates.
Add a unit test in kfilewidgettest.
BUG: 408801
BUG: 401916
BUG: 411353
BUG: 411219
BUG: 409285
BUG: 405461
BUG: 405046
FIXED-IN: 5.63.0