Add guidance on negative tests in HACKING.rst
Add guidance on negative tests in HACKING.rst

Adds guidance on negative tests based on work items from the Newton
design summit in Austin[1].

[1] https://etherpad.openstack.org/p/newton-qa-negative-testing

Change-Id: Ic44b0e17540a5e984006a4a74c8318ad7e74db31

openstack/tempest,Tesora/tesora-tempest,sebrandon1/tempest,openstack/tempest,masayukig/tempest,Juniper/tempest,cisco-openstack/tempest,cisco-openstack/tempest,Juniper/tempest,sebrandon1/tempest,Tesora/tesora-tempest,vedujoshi/tempest,masayukig/tempest,vedujoshi/tempest