Add unit tests for the Draft class

The Draft class had no dedicated tests. Cover the save/retrieve round
trip for whole-page and section edits, the cases where no draft is
written (drafts disabled,

Add unit tests for the Draft class

The Draft class had no dedicated tests. Cover the save/retrieve round
trip for whole-page and section edits, the cases where no draft is
written (drafts disabled, nothing edited), draft deletion, reading a
missing draft, and the constructor's purging of drafts that predate the
current page revision.

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