pgsql: Temporarily instrument postgres_fdw test to look forstatistics

Temporarily instrument postgres_fdw test to look for statistics changes.

It seems fairly hard to explain recent buildfarm failures without the
theory that something is doing an ANALYZE behind our backs.  Probe
for this directly to see if it's true.

In principle the outputs of these queries should be stable, since the table
in question is small enough that ANALYZE's sample will include all rows.
But even if that turns out to be wrong, we can put up with some failures
for a bit.  I don't intend to leave this here indefinitely.

Discussion: https://postgr.es/m/25502.1520277552@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c2c537c56dc30ec3cdc12051f4ea5363aa66d73c

Modified Files
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contrib/postgres_fdw/expected/postgres_fdw.out | 31 ++++++++++++++++++++++++++
contrib/postgres_fdw/sql/postgres_fdw.sql      | 13 +++++++++++
2 files changed, 44 insertions(+)