Use DB fixtures in Queue Processor and systemtests

[Pasarus]

Description

Issue raised by: [developer/user/project requirement]

What?
The tests in the Queue Processor and systemtests don't use any DB fixtures and rely on having a manually migrated sqlite3.db, which makes the testing somewhat dependent on the development state of the app.

It would be a lot better to use fixtures and an in-memory sqlite3 db, to ensure a clean state for each test.