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

src/test/modules/test_oat_hooks/expected/test_oat_hooks.out

@@ -1,3 +1,7 @@
+-- This test script fails if debug_discard_caches is enabled, because cache
+-- flushes cause extra calls of the OAT hook in recomputeNamespacePath,
+-- resulting in more NOTICE messages than are in the expected output.
+SET debug_discard_caches = 0;
 -- Creating privileges on a placeholder GUC should create entries in the
 -- pg_parameter_acl catalog which conservatively grant no privileges to public.
 CREATE ROLE regress_role_joe;

src/test/modules/test_oat_hooks/sql/test_oat_hooks.sql

@@ -1,3 +1,8 @@
+-- This test script fails if debug_discard_caches is enabled, because cache
+-- flushes cause extra calls of the OAT hook in recomputeNamespacePath,
+-- resulting in more NOTICE messages than are in the expected output.
+SET debug_discard_caches = 0;
+
 -- Creating privileges on a placeholder GUC should create entries in the
 -- pg_parameter_acl catalog which conservatively grant no privileges to public.
 CREATE ROLE regress_role_joe;