ResultsSerializer.running_jobs_for_run() and test

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Make check for running jobs part of the loop

Use a new, tested implementation to check for running jobs.

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Merge pull request #134 from ceph/wip-6546

Fix race condition between tests starting and teuthology-results being run