qa/workunits/cephtool/test.sh: fix thrash (ultimate)

Keep the osd trash test to ensure it is a valid command but make it a
noop by giving it a zero argument (meaning thrash 0 OSD maps).

Remove the loops that were added after the command in an attempt to wait
for the cluster to recover and not pollute the rest of the tests. Actual
testing of osd thrash would require a dedicated cluster because it the
side effects are random and it is unnecessarily difficult to ensure they
are finished.

http://tracker.ceph.com/issues/9620 Fixes: #9620

Signed-off-by: Loic Dachary <loic-201408@dachary.org>