Fix kio-jobtest on odd file systems.

Some file systems (i.e. btrfs) report 0 as file size for directories.
That caused this test to fail, as the total file size of the tested
files and directories fell below the tested threshold.

Change the threshold value to only cover the size of the files.
(cherry picked from commit dff93704fbf325dadf0b151f47d2b37d0120eb05)