Refactor database population in tests to be useable also outside of tests (#378)

To work on #378 (closed) I need to add work requests, artifacts and build logs to the UI playground script.

To do that I would need to call TestHelpersMixin methods from it, or reimplement those methods.

This MR moves those methods into a separate class that can also be used outside of unit tests.

To move methods that create files in the file store, it also implements a memory file store to be used in tests, which supports rolling back to a known state for use in setUpTestCase