Add test case showing that commit d0d44049d fixed a live bug.

Add test case showing that commit d0d44049d fixed a live bug.

When I committed d0d44049d (Account for optimized MinMax aggregates
during SS_finalize_plan), I didn't have a test case showing that it
was fixing any reachable bug.  Here is one, based on bug #18465 from
Hal Takahara.  Without the fix, all rows of the result show the same
"min" value, because the aggregate doesn't get recalculated.

Committed despite beta1 release freeze, with the concurrence of
pgsql-release.

Discussion: https://postgr.es/m/18465-2fae927718976b22@postgresql.org
Discussion: https://postgr.es/m/2391880.1689025003@sss.pgh.pa.us