qa/tasks: Add additional wait_for_clean() check in lost_unfound tasks.

qa/tasks: Add additional wait_for_clean() check in lost_unfound tasks.

At the end of the lost_unfound tests add an additional wait_for_clean()
check to ensure that recoveries get enough time to complete before
proceeding and avoid failures down the line. For e.g. failure like
"Scrubbing terminated -- not all pgs were active and clean." is because
recoveries on the PGs did not get sufficient time to complete even though
they were bound to eventually complete.

Fixes: https://tracker.ceph.com/issues/49844
Signed-off-by: Sridhar Seshasayee <sseshasa@redhat.com>
(cherry picked from commit 88df47230b5ad85e95b0be2eca6f5763914b175c)