Add Unit-Tests for JsonSelector

[silvanheller]

Currently, there are Unit Tests available for Cottontail DB in the org.vitrivr.cineast.core.db.cottontaildb package. A good way to familiarize yourself with the internal abstraction layer of Cineast to the Underlying Database would be to add Unit-Tests similar to the ones existing for Cottontail for the JsonSelector. This is rather easy, as the unit tests already contain abstraction layers to make this easier.