test: pass snapshot listings to the catch-up restart budget probe

Main now snapshots systemd unit listings before stopping old processes
and passes them into _restart_systemd_gateway_units_best_effort; the
catch-up budget test and eval call the new two-argument shape while
still asserting the unit's stop+start budget reaches the client timeout.

evals/update_unit_client_budget.py

@@ -43,7 +43,7 @@ try:
     try:
         if catchup:
             failed = []
-            fleet._restart_systemd_gateway_units_best_effort(failed)
+            fleet._restart_systemd_gateway_units_best_effort(failed, list(fleet._systemd_gateway_unit_listings()))
             record.update(failed_units=failed)
         else:
             result = _systemctl_reset_and_restart(cmd, unit)