pgsql: Fix matching check in recovery test 042_low_level_backup

Fix matching check in recovery test 042_low_level_backup

042_low_level_backup compared the result of a query two times with a
comparison operator based on an integer, while the result should be
compared with a string.

The outcome of the tests is currently not impacted by this change.
However, it could be possible that the tests fail to detect future
issues if the query results become different, for some reason.

Oversight in 99b4a63bef94.

Author: Sadhuprasad Patro <b(dot)sadhu(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAFF0-CHhwNx_Cv2uy7tKjODUbeOgPrJpW4Rpf1jqB16_1bU2sg@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/3a7225ed37fc0819f689e17435dc590fd2b8980d

Modified Files
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src/test/recovery/t/042_low_level_backup.pl | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)