Adding sign in scenarios for FXA

This is for issue https://github.com/mozilla/marketplace-tests/issues/553
@krupa requested more scenarios to login for FXA. This will cover it for Desktop
This was a solution a while ago for the issue when marketplace runs at the same time on Sauce and our nodes.
This is also in <code>test_that_checks_the_addition_of_a_review</code>, I haven't really looked over this to find another solution, as it is indeed not needed for tests that add a review.