pgsql: postgres_fdw: Try again to stabilize aggregate pushdown regressi

postgres_fdw: Try again to stabilize aggregate pushdown regression tests.

A query that only aggregates one row isn't a great argument for pushdown,
and buildfarm member brolga decides against it. Adjust the query a bit
in the hopes of getting remote aggregation to win consistently.

Jeevan Chalke, per suggestion from Tom Lane

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/f5d6bce63ceb3c59a964814bb0df5a0648e750e5

Modified Files
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contrib/postgres_fdw/expected/postgres_fdw.out | 58 +++++++++++---------------
contrib/postgres_fdw/sql/postgres_fdw.sql | 4 +-
2 files changed, 27 insertions(+), 35 deletions(-)