[uss_qualifier] factorize uss availability fragment

[RustedBones]

Implement counterpart of the DSS availability read and update fragments in dss/test_step_fragments.py

Reuse those fragments in the USSAvailabilitySynchronization scenario
Update the set_uss_down and set_uss_available fragments to read state version before executing an update to prepare for #1137

Move make_dss_report in dss package to avoid circular imports

[BenjaminPelletier]

I think this maintains parity with the existing implementation, but if we actually expect check.record_failed to interrupt execution below, then it seems like we expect these values never to be used. In that case, I think it would be better to return inside the try and raise a ScenarioDidNotStopError inside the except. That can be a separate-PR follow-up.