tests: fix recent false failure due to a race

tests: fix recent false failure due to a race

* tests/cp/parent-perm-race.sh: This new race introduced in
commit v8.25-5-g632eda5 is quite hard to hit, but is due to
`ls > fifo` doing write()/close()/exit() once `cp` has
open() the source fifo.  Then the subsequent comparison of the
destination file may fail due to the file being missing or empty.
Previously `ls` generated output that was independent of `cp`.
Now we must wait for `cp` to finish before inspecting the
destination file that it wrote.