Further stabilize a postgres_fdw test case.

Further stabilize a postgres_fdw test case.

The buildfarm occasionally shows a variant row order in the output
of this UPDATE ... RETURNING, implying that the preceding INSERT
dropped one of the rows into some free space within the table rather
than appending them all at the end.  It's not entirely clear why that
happens some times and not other times, but we have established that
it's affected by concurrent activity in other databases of the
cluster.  In any case, the behavior is not wrong; the test is at fault
for presuming that a seqscan will give deterministic row ordering.
Add an ORDER BY atop the update to stop the buildfarm noise.

The buildfarm seems to have shown this only in v18 and master
branches, but just in case the cause is older, back-patch to
all supported branches.

Discussion: https://postgr.es/m/3866274.1770743162@sss.pgh.pa.us
Backpatch-through: 14