pgsql: Fix plan instability in the new tuplesort test.

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.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/80ef34fc7075b37fc23f4ab714a5ce60f82400de

Modified Files
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src/test/regress/expected/tuplesort.out | 10 +++++-----
src/test/regress/sql/tuplesort.sql      |  1 +
2 files changed, 6 insertions(+), 5 deletions(-)