Add test scenario validating USS behavior when requesting OVN suffix

[mickmis]

Is your feature request related to a problem? Please describe.
interuss/dss#1119 adds support in DSS for USSs to optionally request OVNs to be set. Testing the behavior of the DSS with regards to that optional extension was implemented in #816, however testing the behavior of USSs using it is still missing. This means the uss_qualifier cannot credit a requirement to USSs that support the feature.

Describe the solution you'd like

• Add to the mock_uss support for this feature
• Add a requirement and test scenario that validates USSs behavior with regards to this feature