Tweak postgres_fdw regression test so autovacuum doesn't change results.

Autovacuum occurring while the test runs could allow some of the inserts to
go into recycled space, thus changing the output ordering of later queries.
While we could complicate those queries to force sorting of their output
rows, it doesn't seem like that would make the test better in any
meaningful way, and conceivably it could hide unexpected diffs.  Instead,
tweak the affected queries so that the inserted rows aren't updated by the
following UPDATE.  Per buildfarm.