If you already have APOC's classes in your app, you can do it the same way that APOC does it, by using its TestUtil.registerProcedure method.
Here's an example from APOC's unit tests, something very similar should work for you.
So in this testing what's happening is that you're creating a temporary in memory Neo4j instance. Much as you'd launch neo4j and need to put your plugin into the plugins directory, this temporary instance has to be told what plugin to load to accomplish your test. That's what's happening here.