Add more unit tests for ThreadGroupManager
Add more unit tests for ThreadGroupManager

this patch adds unit tests for the rest of ThreadGroupManager methods.
Related-Bug: #1288627
Change-Id: I38a0864c649d1693ef4fb5b49d5aeb37632263ca

dims/heat,takeshineshiro/heat,noironetworks/heat,rdo-management/heat,dragorosson/heat,gonzolino/heat,rdo-management/heat,cryptickp/heat,pratikmallya/heat,dims/heat,srznew/heat,rh-s/heat,pshchelo/heat,cwolferh/heat-scratch,NeCTAR-RC/heat,jasondunsmore/heat,steveb/heat,noironetworks/heat,openstack/heat,pshchelo/heat,maestro-hybrid-cloud/heat,pratikmallya/heat,NeCTAR-RC/heat,takeshineshiro/heat,cwolferh/heat-scratch,redhat-openstack/heat,miguelgrinberg/heat,maestro-hybrid-cloud/heat,redhat-openstack/heat,cryptickp/heat,srznew/heat,steveb/heat,gonzolino/heat,rh-s/heat,miguelgrinberg/heat,jasondunsmore/heat,dragorosson/heat,openstack/heat