Create test plan with plan for unit tests and more

This library is in initial phase and the main focus has been in finding a model for the Reactive API for Apache Pulsar.

There are only a few end-to-end tests at the moment.
To add tests, it makes sense to first do a high level test plan and describe the test strategy.

Each "unit" should be listed in the plan and the main use cases to test would be listed too. The units aren't necessary single classes. Unit tests should execute quickly.