we removed an item from the tree while iterating it.

Spotted by flexelint with stricter rules, see follow up commits.

See tree(3bsd)

to be able to do this and see the right object, we need to change the
test case to a private upload directory (defaults to per-vtc working
directory)

to be able to use Lck_AssertHeld()

as of efa1e7d64f8b1a17a02a9fd30df606804ba40a3b

kick the expiry thread