test/modules/test_oat_hooks vs. debug_discard_caches=1

We realized today [1] that it's been some time since the buildfarm
had any debug_discard_caches (nee CLOBBER_CACHE_ALWAYS) coverage.
Sure enough, as soon as Tomas turned that back on, kaboom [2].
The test_oat_hooks test is failing --- it's not crashing, but
it's emitting more NOTICE lines than the expected output includes,
evidently as a result of the hooks getting invoked extra times
during cache reloads.  I can reproduce that here.

Maybe it was a poor design that these hooks were placed someplace
that's sensitive to that.  I dunno.  The only short-term solution
I can think of is to force debug_discard_caches to 0 within that
test script, which is annoying but feasible (since that module
only exists in v15+).

Thoughts, other proposals?

			regards, tom lane

[1] https://www.postgresql.org/message-id/6b52e783-1b32-e723-4311-0e433a5a5a75%40enterprisedb.com
[2] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=avocet&dt=2022-11-18%2016%3A01%3A43