Make stats_ext test faster under cache-clobbering test conditions.

Make stats_ext test faster under cache-clobbering test conditions.

Commit 1eccb9315 added a test case that will cause a large number
of evaluations of a plpgsql function.  With -DCLOBBER_CACHE_ALWAYS,
that takes an unreasonable amount of time (hours) because the
function's cache entries are repeatedly deleted and rebuilt.
That doesn't add any useful test coverage --- other test cases
already exercise plpgsql well enough --- and it's not part of what
this test intended to cover.  We can get the same planner coverage,
if not more, by making the test directly invoke numeric_lt().

Reported-by: Tomas Vondra <tomas@vondra.me>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/baf1ae02-83bd-4f5d-872a-1d04f11a9073@vondra.me