Decide and document how to test actors that interact with HTTP or TCP

**Issue by [rkuhn](https://github.com/rkuhn)**
_Tuesday Dec 09, 2014 at 13:05 GMT_
_Originally opened as https://github.com/akka/akka/issues/16493_

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Code using `StreamTcp().connect(...)` and friends can currently not be mocked, it will always perform TCP connections. This may or may not be a problem depending on how we want to tell users that they shall test their actors:
- No mocking means that you’ll test exactly what is used in production, but it requires all such tests to be end-to-end.
- Being able to reroute the API towards TestProbes or similar means that the test can be kept shorter, with less boiler-plate code and without the need to create a communication partner just for the test procedure.

Start by examining code that uses StreamTcp().connect(...) and the current actor-testing guidance. Compare end-to-end testing with rerouting the API to TestProbes or similar, then document the chosen approach and what users should test.

Written by the indexing model from the issue text.