tests: check osd health in cephtool/test.sh

Add a trivial osd 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.

These checks can be disabled with --no-sanity-check to allow running
some tests that have less requirements than running all the tests.

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