tests: du: avoid false failure in racy test

tests: du: avoid false failure in racy test

* tests/du/move-dir-while-traversing.sh: Expand the work to avoid
a false failure where du completes before the directory is moved.
Also expand the timeout to our more standard 10s to avoid the
"directory mover" being killed before du processes the directory.
This doesn't perceptibly impact the run time of the test.
Reported by Bruno Haible on a CentOS 7 system.