Increase PerformBackupTask unit coverage

With KV Refactor in mind.
* Added tests around empty queue and single package backups.
* Refactored a bit some of the existing tests.
* Moved from mocking BMS to using a real instance and had to adjust a
  few things for this.

Test: atest PerformBackupTaskTest
Change-Id: I0ee3be32c7cbac5ed2cdc2717408749907c15ade