[MERGE] Add PreviewTree to the tree implementation tests

[Aaron Bentley]

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Tim Penhey wrote:
> Does this mean that a PreviewTree can be used as a merge source
> now?

No, though supporting this sort of thing is one of my goals for this work.

Right now, we are only testing that if you don't make any changes, the
PreviewTree behaves the same as the base tree.  I am certain that it
does not behave correctly in all cases if you do make changes.

Aaron
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