write contains* methods. Test output ->

-> Test output fine but we need a bigger test set!

$ java TestIntSets
Run tests on TreeIntSet:
add 7 to the set
add 9 to the set
add 7 to the set again!
print the set contents: 7, 9
verbose test whether set contains 9:
	contains checks 9 against 7
	contains checks 9 against 9
	Result set does contain 9
verbose test whether set contains 19:
	contains checks 19 against 7
	contains checks 19 against 9
	Result set does not contain 19