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_17_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/61b5fa029a432e6c5f516c633b5467d4cc55f264

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