Update unit test debug instructions
Update unit test debug instructions

Since we've moved to stestr, the test debugging instructions we had
published in our documentation do not actually work anymore. This
updates those instructions to show a couple possible methods that can be
used to perform test debugging.

Change-Id: I9946da0c0d9aa247ff820ee6c02e57bf86a2747d
Signed-off-by: Sean McGinnis <961eeb09b6d45516665e15b82e013282c4faf575@gmail.com>

mahak/cinder,openstack/cinder,mahak/cinder,openstack/cinder