Add end-to-end tests with multiple senders and receivers

[gavv]

TestEnd2End_Default is a simple integration test that writes samples to sender, reads them from receiver, and verifies the received stream.

This test uses a single sender and a single receiver, however other combinations are supported too:

• many senders, one receiver
• one sender, many receivers
• many senders, many receivers

These combinations are supported using slots. You may create multiple slots on a sender, and connect each slot to different receiver. You can create multiple slots on receiver, and connect senders to different receiver slots. Finally, you can connect multiple senders to the same slot of receiver, but in this case all senders should use the same protocol.

We should cover all these combinations with tests (feel free to suggest better names):

• TestEnd2End_Connections/one_sender_two_receivers

  Two receivers (each with one slot), and one sender with two slots, connected to two receivers.

• TestEnd2End_Connections/one_receiver_one_slot_two_senders

  One receiver with one slot, and two senders connected to the same receiver slot.

• TestEnd2End_Connections/one_receiver_two_slots_two_senders

  One receiver with two slot (ideally with two different protocols), and two senders, each connected to own slot.

• TestEnd2End_Connections/two_receivers_two_senders

  Two receivers, each with one slot, and two senders, each with two slots, one slot connected to first receiver, and another slot connected to second receiver.

TestEnd2End_Default and TestEnd2End_Connections should share the code for sending and receiving samples, so that the algorithm described in #101 would be used for both of them.