LPS-32535 Staged Model data handlers testing

Hey Julio,

Here goes the long awaited testing for the staged model data handlers.

As we discussed earlier there is a base test class, and there should be new test classes extending the base for different staged models. In this batch @KocsisDaniel and I were creating two test cases: one for BookmarksFolder and one for BookmarksEntry.

One note on the testing: We have only been working on the staged model data handler tests, and not on the portlet data handler tests. The reason is that we are planning to get rid or at least hugely refactor the portlet data handlers.

Thanks,

Máté