test v23-feeDistributor re-subscribes to timer notifier when v5-timer is upgraded

[warner]

As part of #8499 (comment) (look for v5-timer-v23-feedistributor-v48-vaultfactory), we're investigating how v23-feeDistributor will react when the upstream v5-timer is upgraded.

v23-feeDistributor has code which talks to the timer service and creates a notifier during startDistributing. The timer service provides durable notifiers, and this subscriber code correctly uses observeNotifier to react to vat-timer being upgraded (and the individual promises being disconnected).

(I suspect this is all working, but if not, it would be a blocker to upgrading v5-timer. Someone who knows what is tested and what is not may be able to trivially close this ticket)

The task is to test this tolerance of upgrading v5-timer, probably through a3p:

• setup a feeDistributor vat, which will establish the timer loop
• upgrade v5-timer
• assert that the feeDistributor has re-subscribed to the timer service
  • perhaps by allowing time to advanace and then observing the feeDistributor reacting to the passage of time correctly