Further stabilize a postgres_fdw test case.

Further stabilize a postgres_fdw test case.

This patch causes one postgres_fdw test case to revert to the plan
it used before aa86129e1, i.e., using a remote sort in preference to
local sort.  That decision is actually a coin-flip because cost_sort()
will give the same answer on both sides, so that the plan choice comes
down to little more than roundoff error.  In consequence, the test
output can change as a result of even minor changes in nearby costs,
as we saw in aa86129e1 (compare also b690e5fac and 4b14e1871).

b690e5fac's solution to stabilizing the adjacent test case was to
disable sorting locally, and here we extend that to the currently-
problematic case.  Without this, the following patch would cause this
plan choice to change back in this same way, for even less apparent
reason.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/2551253.1766952956@sss.pgh.pa.us