pgsql: Change recently added test code for stability

Change recently added test code for stability

The test code added with ff9f111bce24 fails under valgrind, and probably
other slow cases too, because if (say) autovacuum runs in between and
produces WAL of its own, the large INSERT fails to account for that in
the LSN calculations.  Rewrite to use a DO loop.

Per complaint from Andres Freund

Backpatch to all branches.

Discussion: https://postgr.es/m/20211013180338.5guyqzpkcisqugrl@alap3.anarazel.de

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/79c7fe1af82d42211baf37e8dc865288b251cc6d

Modified Files
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src/test/recovery/t/026_overwrite_contrecord.pl | 39 ++++++++++++++-----------
1 file changed, 22 insertions(+), 17 deletions(-)