Add duplex test

[sake92]

While working on Windows fixes for Mill, I thought that duplex mode was not working properly.
Mill writes/reads to socket simultaneously from different threads.
But seems like I was wrong.
Anyway, this test should be useful to cover it..

[eed3si9n]

Just noticed this. Thanks!

[eed3si9n]

Hmm. I am not sure if it's working actually. CI failed immediately upon merge:

https://github.com/sbt/ipcsocket/runs/7058379356?check_suite_focus=true

[client] got a message: hello12
java.lang.InterruptedException: sleep interrupted
	at java.lang.Thread.sleep(Native Method)
	at org.scalasbt.ipcsocket.duplex.Receiver.run(Receiver.java:28)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)
[server] sending msg: 13
[server] sending msg: 14
[error] Test org.scalasbt.ipcsocket.duplex.DuplexTest.testDuplexCommunication failed: expected:<15> but was:<13>, took 18.014 sec

[eed3si9n]

More sleeping seems to have fixed it #25