pgsql: Further stabilize postgres_fdw test.

Further stabilize postgres_fdw test.

The queries involving ft1_nopw don't stably return the same row
anymore. I surmise that an autovacuum hitting "S 1"."T 1"
right after the updates introduced by f61db909d/5843659d0 freed
some space, changing where subsequent insertions get stored.
It's only by good luck that these results were stable before,
though, since a LIMIT without ORDER BY isn't well defined,
and it's not like we've ever treated that table as append-only
in this test script.

Since we only really care whether these commands succeed or not,
just replace "SELECT *" with "SELECT 1".

Report: https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=crake&dt=2021-06-23%2019%3A52%3A08

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/802177090992511c610804da54a4603d4f50c594

Modified Files
--------------
contrib/postgres_fdw/expected/postgres_fdw.out | 24 ++++++++++++------------
contrib/postgres_fdw/sql/postgres_fdw.sql | 12 ++++++------
2 files changed, 18 insertions(+), 18 deletions(-)