[test] `test_routing_manager` to check duplicate PIO/RIO prefixes (#8431)
[test] `test_routing_manager` to check duplicate PIO/RIO prefixes (#8431)

This commit updates the `RoutingManager` unit test to check that
there are no duplicate PIO/RIO prefixes in the emitted RA message.

It also updates the `TestSavedOnLinkPrefixes()` to check behavior
after device resets/restarts while it was advertising its local
on-link prefix or deprecating it.
openthread/openthread,openthread/openthread,openthread/openthread,openthread/openthread