Test suite: strengthen function _cluster_is_healthy().

Test suite: strengthen function _cluster_is_healthy().

If there's a chance that "ctdb status -Y" can return 0 but print
garbage then this function might return a false positive.

So, we do 2 things:

* Redirect stderr to >/dev/null rather than looking at it.  This
  minimises the chance that we will see garbage.

* Since we need at least 1 good line to decide the cluster is healthy,
  we sanity check each line to esnure it starts with :[0-9].

Signed-off-by: Martin Schwenke <martin@meltin.net>