pgsql: Stabilize postgres_fdw regression tests.

Stabilize postgres_fdw regression tests.

The new test cases added in commit 8bf58c0d9 turn out to have output
that can vary depending on the lc_messages setting prevailing on the
test server. Hide the remote end's error messages to ensure stable
output. This isn't a terribly desirable solution; we'd rather know
that the connection failed for the expected reason and not some other
one. But there seems little choice for the moment.

Per buildfarm.

Discussion: https://postgr.es/m/18419.1500658570@sss.pgh.pa.us

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/90877d28e62f362fafa5bc3f0c3b2f7b985123b4

Modified Files
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contrib/postgres_fdw/expected/postgres_fdw.out | 5 +++--
contrib/postgres_fdw/sql/postgres_fdw.sql | 3 +++
2 files changed, 6 insertions(+), 2 deletions(-)