Added some tests for isOK() and test the add method in board.
Merge remote-tracking branch 'origin/master'
Merge branch 'master' of https://github.com/Yisheng-G/cs61bl-proj1

Conflicts:
	Board.java
Merge branch 'master' of https://github.com/Yisheng-G/cs61bl-proj1

isOK conflicts

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