Bug 993114 - [marketplace-tests-gaia] Add a test to purchase an app using Zippy

This includes a commit from issue #22 as some of the same things were needed, so that should be merged first and then I'll rebase this if necessary.

This is also quite a bit of duplication in this test from the other test, and there is some more cleanup/refactoring to do of the code in this repo, but I wanted to get this coverage in for now, and then deal with cleaning the other stuff up later.