[BUGFIX] Add test coverage for EntityResource, Routes and ResourceManager

We are missing tests for some classes.

We need to add test coverage for the methods in the EntityResource, the Routes and the ResourceManager classes.

I suggest adding Kernel testing to make sure that the individual and the collection methods are returning as expected under different scenarios.
I suggest adding Unit testing to make sure that the routes are created for the different bundles and entity types.
I suggest adding Unit testing to make sure that the resource config for the different bundles are available and discovered.