Add more thorough tests for protocol and general behavior

[marcelog]

We're currently missing tests that exercise all the code and behavior involved:

1. Supervision tree
2. Accepting connections and receiving traffic for the different protocol versions.
3. KPL aggregated records (as sent by KCL)
4. Records that are neither KPL/Custom KPL (as sent by KCL)

We do have some tests that actually run some parts of the code, but a good set of integration tests would come in handy (to test incoming data from KCL, how the code behaves and interacts with our users).

Something that can be used for this is to have a small tool built in our tests that will actually connect to the erlmld server, send traffic, and assert the behavior at different parts of the code. In addition, creating some implementations of the erlmld behaviors to make those assertions and test (between other things) checkpointing, etc.