This extends the existing the existing scroll position test by navigating to a second page and then checking that after closing and restoring that tab, the scroll position is restored not only for the current history entry, but after going back as well.
MozReview-Commit-ID: Ddig1Mfo5rz
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extra : rebase_source : 20bdc1f116b27f7386a9b7d1cdc0ad383b21b99c