pgsql: pg_amcheck: Minor test speedups

pg_amcheck: Minor test speedups

Freezing the relation N times and fetching the tuples one-by-one isn't that
cheap. On my machine this reduces test times by a bit less than one second, on
windows CI it's a few seconds.

Reviewed-by: Mark Dilger <mark.dilger@enterprisedb.com>
Discussion: https://postgr.es/m/20230309001558.b7shzvio645ebdta@awork3.anarazel.de

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/a4f23f9b3cddb86137a3b1683dcbe49d8b41ea7b

Modified Files
--------------
src/bin/pg_amcheck/t/004_verify_heapam.pl | 33 ++++++++++++++-----------------
1 file changed, 15 insertions(+), 18 deletions(-)