Positive filtering which steps can be skipped.

@wedaly @dianakhuang 

We should only allow certain steps to be skipped. I had noticed that you could put the skip parameter on the payment step and get to payment confirmation. 

This uses a positive filter to check which step you are trying to skip.

Test added to ensure you cannot skip the payment step (tests already exist for the positive steps)
Maybe update this to say something about the step we expect to be on?  It took me a few moments looking at this test to realize why this was verifying the 'cannot skip' scenario.