xfail test_user_can_go_back_from_settings_page on prod as back button for settings has not made it to prod yet

Unfortunately there is no open bug for this in bugzilla. However, `marketplace-tests` and `gaia-ui-tests` both caught that the home icon on settings page has now been replaced by a back button.

This can be observed on `dev`. However, the change has not made it to `prod` yet.