Rewrite KStreamPeekTest at processor level avoiding driver usage

Hi,
as already done for the KStreamPrintTest we could remove the usage of KStreamTestDriver even in the KStreamPeekTest and testing it at processor level not at stream level.
My proposal is to :

• create the KStreamPeek instance providing the action which fill a collection as already happens today
• testing for both forwardDownStream values true and false
• using the MockProcessorContext class for overriding the forward method filling a streamObserved collection as happens today forwardDownStream is true; checking that the forward isn't called when forwardDownStream is false (so the test fails)

Thanks,
Paolo