test_(dis)associate_by_non_existing_security_group_name missing stub

test_(dis)associate_by_non_existing_security_group_name() is passing because
it is not finding the instance which also returns HTTPNotFound not because
it cannot find the security group. This patch adds the missing stub so
that it it checks for the non existing security group.

Fixes bug 1120832

Change-Id: I67a2b82ac329193c2438038719f8c052de8081a3
Update docs about testing.

Add an entry to the HACKING file about testr. While in there, noticed a
reference to the now-defunct nova/testing dir. Fixed that, moved the testing
README into nova/tests and remove the nova/testing dir.

Change-Id: Ibf6fb82658ba73eee9123fa53b340d0b72afb292
Merge "Update docs about testing."
Merge "test_(dis)associate_by_non_existing_security_group_name missing stub"