Add fixtures to marketplace-tests

Automatically log in to developer hub as needed
Split developer hub tests into two files
Create and automatically delete test apps as needed
Update requirements.txt to point to newest version of API client
Xfail test_that_a_screenshot_can_be_added due to newly uncovered bug

This is ready for review. I plan to address the issue of using the API in pieces. I have started with the developer hub tests that *do not* submit apps, as the submission tests need to use the UI anyway. My apologies that it is so big, but I wanted to do a bit of cleanup too.

Reviews welcome!