Increase PerformBackupTask unit coverage 2

With KV Refactor in mind.
* Split empty queue test into multiple tests.
* Tests around packages failing backup pre-conditions.
* Added verifications for event logging.
* Renamed some tests to better reflect important parts of it.
* More tests about one package.
* Introduced PackageData to help mock packages, much like TransportData.
* Some small refactors.

Test: atest PerformBackupTaskTest
Change-Id: I23aba2b26e82f8643cc1c90f9408510ab34045fd