pgsql: Fix unstable regression test added by commits 59b71c6fe et al.

Fix unstable regression test added by commits 59b71c6fe et al.

The query didn't really have a preferred index, leading to platform-
specific choices of which one to use.  Adjust it to make sure tenk1_hundred
is always chosen.

Per buildfarm.

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/fa39617a64d4134c63b3f62ae12b0268542a79e5

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