Bug 997951 - [marketplace-tests-gaia] Clean up and bolster some tests which are failing intermittently

Add compatibility with current gaiatest

Note that this will require a new release of gaiatest which includes a new release of marionette, so it is waiting on:
https://bugzilla.mozilla.org/show_bug.cgi?id=1002541 and 
https://bugzilla.mozilla.org/show_bug.cgi?id=1001461