Split security group tests

Split up the security group tests between compute and network APIs
into separate files in preparation for reworking the compute (nova-net)
implementations to deal with the upcoming removal of deprecated nova-net
support in novaclient 8.0.0.

No code changes are intended here, just splitting two files into four.

Change-Id: I3fbdde45f593a30de545ddd11e319a4d6f900b18

Author: Dean Troyer