randomize test cases where multiple socket connections are involved

[RoadRunnr]

trafficstars

The test cases always processed the recv statemtn in the same order as the original outgoing message and used the same message for all clients. This had the potential to miss problems where message where returned to the wrong client. It now send a diffent message to each peer and checks that it did recieve the correct message back.

[hintjens]

Travis failed, shall I merge this anyhow?

[RoadRunnr]

I've used a function in the test suite that is not available on Erlang R15. The test suite update is not critical, so let's no merge it yet. I'll update the pull request tomorrow.