Fix the ThreadMgr mocking in test_stack_create_max_unlimited
Fix the ThreadMgr mocking in test_stack_create_max_unlimited

use the helper method _test_stack_create() as it correctly mocks
the ThreadMgr. Otherwise this runs in the background and the test
deletes the stack while it is running in the background.

Change-Id: Ia64a09c93cf5414b0943fc9495c435355b5645fc
Closes-bug: #1432936

jasondunsmore/heat,jasondunsmore/heat,maestro-hybrid-cloud/heat,gonzolino/heat,srznew/heat,steveb/heat,dims/heat,dims/heat,cryptickp/heat,gonzolino/heat,srznew/heat,rh-s/heat,dragorosson/heat,rh-s/heat,maestro-hybrid-cloud/heat,takeshineshiro/heat,dragorosson/heat,miguelgrinberg/heat,cwolferh/heat-scratch,openstack/heat,openstack/heat,pratikmallya/heat,cryptickp/heat,takeshineshiro/heat,steveb/heat,noironetworks/heat,pratikmallya/heat,miguelgrinberg/heat,noironetworks/heat,cwolferh/heat-scratch