contrib/sepgsql regression tests have been broken for months

I tried to run contrib/sepgsql's regression tests today, and was
rather astonished when they failed. Investigating, there are
some context lines like "LINE 1: ALTER TABLE regtest_table_4
ALTER COLUMN y TYPE float;" in the test output that were not there
before. A bit of bisecting showed that the change happened with

65281391a937293db7fa747be218def0e9794550 is the first bad commit
commit 65281391a937293db7fa747be218def0e9794550 (HEAD)
Author: Michael Paquier <michael(at)paquier(dot)xyz>
Date: Mon Jan 27 13:51:23 2025 +0900

Print out error position for some ALTER TABLE ALTER COLUMN type

So, okay, that's a perfectly respectable thing to do, and I can't
really fault Michael or Jian for not having tested its effects on
sepgsql. But how come it took this long to notice?

I think that rhinoceros is the only BF member testing with
--with-selinux. Looking at its logs, it is running the sepgsql tests
(as a custom module) in v17 and before, but not in v18 or HEAD.
I suppose that this is a consequence of trying to rely on the
TAP-test infrastructure that was installed by aeb8ea361 (just a few
days before the aforesaid change, as luck would have it). That TAP
test does work for me, but it does not run on rhinoceros because
(1) there's no --enable-tap-tests in its configure command, and
(2) it doesn't set up environment variable PG_TEST_EXTRA to include
"sepgsql".

Anyway, we seem to need the attached in v18 and HEAD,
and we really ought to get BF coverage going again.

regards, tom lane