Bug 1013918 - [marketplace-tests-gaia] Use the Marketplace-dev app that is already installed on v1.3

All the tests pass locally for me on v1.3
@zacc Do you mind taking a quick look at this. With the packaged app Marketplace is now in a frame that is nested 2 levels deep. I had to do this to be able to get to the Marketplace app. It works fine, but I was wondering if you have any opinions/concerns about it?