Improve the tests with Snowplow Micro

[mscwilson]

The tests currently mock out the sending of events. A more thorough and realistic set of tests would be to use the testing pipeline Snowplow Micro. This has been done for the Snowplow React Native tracker.

Also, the structure of the current tests could be improved. There are not many unit tests, and the integration tests are all together in one long and confusing file. The spec_helper.rb file monkey-patches in some methods for the Emitter class, as well as two class variables. This means that the tests are not completely isolated from each other.