1. added official code of test case for join in the autograder
	(usage: "java nachos.machine.Machine -- nachos.ag.Join -#
	joinAfterFinish=true")
	
2. added our own test case for communicator in the autograder
	(usage: "-- nachos.ag.CommunicatorTest -# Speaker1=27,Speaker2=28" 	27
and 28 can be any number)

3. removed our own test cases for join() and Communicator from
	KThread.selfTest() since we made the test case thru the autograder