pgsql: Use installed postgresql.conf.sample for GUC sanity TAP test

Use installed postgresql.conf.sample for GUC sanity TAP test

The current code looks for the sample file in the source directory, but
it seems better to test against the installed sample file.

Backpatch to release 15 where the test was introduced.

Discussion: https://postgr.es/m/73eea68e-3b6f-5f63-6024-25ed26b52016@dunslane.net

Reviewed by Tom Lane, Alvaro Herrera, Michael Paquier.

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/0086ee356fd9fd0ced2f21e6b31031cb28b4a64a

Modified Files
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src/test/modules/test_misc/t/003_check_guc.pl | 7 +++----
1 file changed, 3 insertions(+), 4 deletions(-)