A TestBatchSource is a BatchSource that the user can fill with events for testing. This makes it possible to write a unit test without connecting to a Ledger.

• Add a TestBatchSource.
  Spin off: Investigate how easy it would be to store a stream from a ledger as a 'protobuf file', which can then be replayed as a TestBatchSource. This could be used for testing specific input from a test ledger or even some data from production. The file could possibly be generated by running a projection into (hypothetical) file actions (with a projection that just takes the event directly and persists to file)