[uss_qualifier/scenarios/netrid] Factor away test step fragment for flight presence from scenario

[mickmis]

Stacked on top of #861, please review last commit only.

This factors away from the RID nominal_behavior scenario as test step fragment the 'flight presence' checks.
Those checks are reused across DP and SP polling, and there are some slight differences between them:

• Premature flight check: no change
• Missing flight check: the SP check covers more requirements than the DP one. However IMO it looks appropriate to have all those requirements covered for both SP and DP (NET0710,1 and NET0340 added on top of NET0610).
• Lingering flight check:
  • for v19: both SP and DP had NET0270 as requirement
  • for v22a: SP has NET0270, and DP has NET0260, but with an issue: the requirement is not linked thus not covered by the check
  • I've decided to change all those by the requirement NET0260,NearRealTime which IMO looks appropriate here, and extends the requirements coverage. Please LMK if you think that is not OK.