pg_waldump: Relax LSN comparison check in TAP test

pg_waldump: Relax LSN comparison check in TAP test

The test 002_save_fullpage.pl, checking --save-fullpage fails with
wal_consistency_checking enabled, due to the fact that the block saved
in the file has the same LSN as the LSN used in the file name.  The test
required that the block LSN is stritly lower than file LSN.  This commit
relaxes the check a bit, by allowing the LSNs to match.

While on it, the test name is reworded to include some information about
the file and block LSNs, which is useful for debugging.

Author: Andrey Borodin <x4mmm@yandex-team.ru>
Discussion: https://postgr.es/m/4226AED7-E38F-419B-AAED-9BC853FB55DE@yandex-team.ru
Backpatch-through: 16