Fix plan instability in the new tuplesort test.

Andres Freund <andres@anarazel.de>

Commit: 80ef34fc7075b37fc23f4ab714a5ce60f82400de
Author: Andres Freund <andres@anarazel.de>
Date: 2019-11-14T00:36:31Z
Releases: 13.0
Fix plan instability in the new tuplesort test.

At least buildfarm member florican doesn't use a material node above a
sort in the mark/restore case. As material is not intended to be
tested with that query, disallow.

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