Broker pallet: `RegionDropped` event fix & additional tests (#1609)

This PR includes the following fix:
- [x] The `duration` is always set to zero in the `RegionDropped` event.
This is fixed in this PR.

Also added some additional tests to cover some cases that aren't covered
:
- [x] Selling a partitioned region to the instantaneous coretime pool.
- [x] Partitioning a region after assigning it to a particular task.
- [x] Interlacing a region after assigning it to a particular task.