Extend general guidelines of conformance tests wrt to regression/unit tests

[martinmo]

Extend the guidelines created in #154 with information on regression/unit tests for the conformance tests (see SovereignCloudStack/standards#447 (comment)).

For completeness, the guidelines should also mention that the conformance checks should be executable with the least privileges possible, i.e., non-admin credentials for OpenStack and Kubernetes APIs and of course the scripts should never need root priveleges (this is already somewhat documented/standardized with scs-0004-v1).