Notification index checks are too strict

[BenjaminPelletier]

Observed behavior
Enabling time-based notification index behavior on the DSS causes some checks to fail.

Test check

• New subscription has a notification index of 0 check
• Create subscription response content is correct check
• Newly created subscription has a notification_index of 0 check
• Mutated subscription has a notification_index of 0 check
• Returned notification index is 0 if present check
• Subscription[P] notification count is properly synchronized with all DSS check (note: the means of synchronization using time-based notification indices is subtle and the justification for this requirement will need to be clear)
• v22a versions of above

Difference from expected behavior
ASTM F3548-21 does not require the behavior expected by the current checks (e.g., that notification_index initially be set to 0). These checks should be revised to only fail if an ASTM F3548-21 requirement is actually violated, or else removed.

Additional context
Affects release of DSS v0.23.0