Trivial: Add running tests section to HACKING.rst
Trivial: Add running tests section to HACKING.rst

The HACKING.rst testing section is added to point out
that we use stestr now instead of testr.

Change-Id: Ic2ef2a70134dacb79dedbbc75f581029e91d0eb6

stackforge/tacker,stackforge/tacker,openstack/tacker,openstack/tacker,openstack/tacker