Replace AbstractMongoTest with JUnit Externalresource

I went to use test. AbstractMongoTest and realized that it was broken and nothing actually used it anyway. So I would like to replace it with a JUnit ExternalResource.

I also noticed that there are several versions of AbstractMongoTest in different places that are not all the same. If this change is accepted, I will make a story to replace all the copy/paste versions with the new ExternalResource.
How about caching a client instead of creating a new one each time?  I assume unit tests might just use this method in-line instead of saving a local copy.