qa/workunits/cephtool/test.sh: wait longer in ceph_watch_start()

qa/workunits/cephtool/test.sh: wait longer in ceph_watch_start()

"ceph --watch-debug" and "ceph tell mon.foo version" could connect
to different monitors, and there is chance that "ceph --watch-debug"
is not connected yet when "ceph tell" completes, and hence the former
fails to collect the cluster log including the "ceph tell" related
message. this renders test_mon_tell() unreliable. so, in
ceph_watch_start(), we should wait until the "ceph" cli connects to the
monitor and receives messages from it.

Fixes: #14910
Signed-off-by: Kefu Chai <kchai@redhat.com>