test/osd-scrub-dump.sh: fix scrub chunk size

test/osd-scrub-dump.sh: fix scrub chunk size

The test performs shallow scrubs, intentionally using small chunk
sizes to allow dump commands time to check specific details.
Following commit ffda64119fc6165cebffd0b26619ad47b13daf51
(PR#44749), shallow scrubs chunks are controlled by a separate
configuration parameter. This PR fixes the test to use the
correct parameter.

An additional minor change is an adjustment to the test loop sleep time:
it is now reduced to guarantee that a dump followed by a counter
increase will be performed in more-or-less the scrubs frequency.

Fixes: https://tracker.ceph.com/issues/58797
Signed-off-by: Ronen Friedman <rfriedma@redhat.com>