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 code is causing a significant delay in running the test, so I don't think we should delay this. Fixing this issue should be part of this pull request, as we don't want to be adding code to the repo that we know is flawed.