create Testing-only mock ES client/search engine

[whatisgalen]

Tests that write or read data from an ES index can fail because of the lag time in indexing. Furthermore, being able to know and manage state in tests doesn't really apply to the ES index. We should still be able to test code that ultimately sends a request to ES but that search backend should work synchronously in tests and should be easily maintainable as part of the test class tearDown method.