tests: rewrite batching test in a less racy way
tests: rewrite batching test in a less racy way

The current test fails from time to time, this one should always work
(of course).

Change-Id: If6ec721bdac0fcb5c679018f503610ef513d1b68

ityaptin/ceilometer,openstack/ceilometer,ityaptin/ceilometer,openstack/ceilometer