LPS-33456 Tests for DL StagedModelDataHandlers

Hey Julio,

Here goes some of the test classes for document library SMDHs. I've excluded the repository related ones, it requires more implementation, but in the meantime I thought I'll commit these since they are finished now. And I've discovered some bugs as well, so it's nice to fix those anyway.

I'll continue working on the Repository related tests, and will send those soon when ready. Will be handled in a separate ticket.

Thanks,

Máté

(@KocsisDaniel reviewed the changes)