pgsql: Fix test case for 'outer pathkeys do not match mergeclauses'fix

Fix test case for 'outer pathkeys do not match mergeclauses' fix.

Commit 4bbf6edfbd5d03743ff82dda2f00c738fb3208f5 added a test case,
but it turns out that the test case doesn't reliably test for the
bug, and in the context of the regression test suite did not because
ANALYZE had not been run.

Report and patch by Etsuro Fujita.  I added a comment along lines
previously suggested by Tom Lane.

Discussion: http://postgr.es/m/5A6195D8.8060206@lab.ntt.co.jp

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d397f558d5552b01643f000d4db7e0960d76241f

Modified Files
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contrib/postgres_fdw/expected/postgres_fdw.out | 77 +++++++++++++-------------
contrib/postgres_fdw/sql/postgres_fdw.sql      | 13 +++--
2 files changed, 48 insertions(+), 42 deletions(-)