Fix "flaky" unit tests

[BPerlakiH]

After double checking our build issues, I realised that some unit-test results, are heavily dependent on User Defaults, which is a key/value storage by Apple, which is persisted across sessions.
By running the same Unit-tests several times (repeatedly without building) some of them might fail.

Proposed solution:

• Wrap our user-default calls for those failing tests
• Use a test instance instead, that won't persist the results
• Also make sure that the created test db entries are cleaned-up properly

[kelson42]

This does impact only the devs IMHO, portponing.