Should use lambda instead of Proc.new

Any tests of Observable.generate had not effect, because
return of Proc.new broke the test method that is caller.

However, return of the lambda should be break its block.
Add assertion message for more detail
Fix a testcase test_generate_raise_iterate, the Observable.generate call the on_next with initial value at first.
Fix a testcase test_generate_dispose, the argument of TestScheduler#configure was wrong
Add the validation for TestScheduler#configure