Ensure all Unit Tests nullify any created resources, related to DS-2080

This PR is more minor cleanup of all our Unit Tests to ensure they are nullifying any resources they create. Setting resources to null ensures that JUnit will clean them up from memory.  

While DS-2080 fixed up a lot of our Unit Tests, this PR cleans up a few that were overlooked, especially the Unit Tests for dspace-services and dspace-oai.

Assuming Travis gives this the OK, I'll merge this ASAP. It's nothing controversial or fancy.