pgsql: Disable debug_discard_caches in test_oat_hooks test.

Disable debug_discard_caches in test_oat_hooks test.

The test output varies when debug_discard_caches is enabled,
because that causes extra executions of recomputeNamespacePath.
Maybe putting a hook in that was a bad idea, but as a stopgap,
just turn off debug_discard_caches in this test.

Per buildfarm (now that we have debug_discard_caches coverage
again).  Back-patch to v15 where this module was added.

Discussion: https://postgr.es/m/2267406.1668804934@sss.pgh.pa.us

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/3efc82e28951c6eee24aead056b8478cb424af2e

Modified Files
--------------
src/test/modules/test_oat_hooks/expected/test_oat_hooks.out | 4 ++++
src/test/modules/test_oat_hooks/sql/test_oat_hooks.sql      | 5 +++++
2 files changed, 9 insertions(+)