elasticsearch embedded in Node.js
There are no existing mock libraries for Elasticsearch and Node at the moment. If you know exactly what queries you are going to be performing, you could use a mocking library to mock the entire ES client and assert simple behaviors.
What I'd recommend, however, is actually running ES in a development environment for testing. I find that mocking external services is a pretty tricky thing to do in general, and your tests will likely be much more robust / trustworthy if running against an actual instance of the server.