pgsql: Add test for session_preload_libraries and parameter permissions

Add test for session_preload_libraries and parameter permissions checks.

We weren't exercising the session_preload_libraries option in any
meaningful way. auto_explain is a good testbed for doing so, since
it's one of the primary use-cases for session_preload_libraries.
Hence, adjust its TAP test to load the library via
session_preload_libraries not shared_preload_libraries. While at it,
feed test-specific settings to the backend via PGOPTIONS rather than
tediously rewriting postgresql.conf.

Also, since auto_explain has some PGC_SUSET parameters, we can use it
to provide a test case for the permissions-checking bug just fixed
by commit b35617de3.

Back-patch to v15 so that we have coverage for the permissions issue
in that branch too. To do that, I back-patched the refactoring
recently done by commit 550bc0a6c.

Dagfinn Ilmari Mannsåker and Tom Lane

Discussion: https://postgr.es/m/CABwTF4VEpwTHhRQ+q5MiC5ucngN-whN-PdcKeufX7eLSoAfbZA@mail.gmail.com

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e1bd4990b2b599b2de7ae531516660c86fbb910b

Modified Files
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contrib/auto_explain/t/001_auto_explain.pl | 125 +++++++++++++++++++++++++----
1 file changed, 108 insertions(+), 17 deletions(-)