tests: check cluster health in cephtool/test.sh

Add a trivial cluster health test at the beginning of each group of
tests. When facing an intermittent failure, it is difficult to diagnose
if the cluster appears to be missing an OSD but there is no indication
as to when the OSDs were last up.

The tests are now only run after all OSDs are up.

Signed-off-by: Loic Dachary <loic-201408@dachary.org>