Have tests confirm that the server is using threads

[NicMcPhee]

I don't have any idea how to check that the client is actually using threads, TBH, but we can test that the server is using threads by having two or more clients talk to it at once and confirm that they can all have a conversation at the "same time".

I'm not sure how to avoid problems of the tests hanging if people don't get the threading right, though.

[NicMcPhee]

@helloworld12321 shared a really good idea here: have two clients connect to a server and confirm that they are both able to communicate in an interleaved fashion.

This only checks that the server is threaded, but that's better than nothing. I still don't know any way to tell that the client is threaded, though.

[helloworld12321]

Oh, that "tests might hang" thing could be a real issue—it looks like bats doesn't have the ability to set timeouts.

(See: bats-core/bats-core#396)