Improved test coverage

- Got rid of JSXTransformer in test suite since it is not needed
- Simplified firebaseRef validation (we ran into a Node bug with this
old validation in GeoFire so we should change it here as well)
- Added a bunch of tests to improve code coverage
- Simplified existing tests
binding the test to an error string seems kind of brittle. Is there any kind of error code or anything we can check for instead?