pgsql: Speed up pg_waldump TAP test

Speed up pg_waldump TAP test

Several invocations decoded and printed the full WAL range even though
the test only needs to confirm that a command form works or that
decoding reaches a specific error. Limit those checks to one record,
and start the fall-off-the-end checks near the end of the generated WAL.

This change reduces the IPC overhead overall, particularly on Windows,
without reducing coverage.

I am usually hesitant to backpatch such changes as this is only an
improvement, but the gains are too good in terms of IO and runtime, for
both the CI and the buildfarm. Based on the numbers provided, the CI
takes 30% less time to run the test with this change on Windows (worst
case shown on the lists). These tests have been introduced in
96063e28366b.

Author: Sehrope Sarkuni <sehrope(at)jackdb(dot)com>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Reviewed-by: Nazir Bilal Yavuz <byavuz81(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAH7T-araSEdsNpxiKaMOW8kr_ZHhekCLHx5yzm2ksq4FdgULbA@mail.gmail.com
Backpatch-through: 17

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e075a10719c1a426d4602ffd4c52d5ee79b084b1

Modified Files
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src/bin/pg_waldump/t/001_basic.pl | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)