pgsql: Make stats_ext test faster under cache-clobbering test condition

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(at)vondra(dot)me>
Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/baf1ae02-83bd-4f5d-872a-1d04f11a9073@vondra.me

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8d61228717e619b90b8ebd1d219d006b920e00e5

Modified Files
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src/test/regress/expected/stats_ext.out | 10 +++-------
src/test/regress/sql/stats_ext.sql | 11 +++--------
2 files changed, 6 insertions(+), 15 deletions(-)