pgsql: Detach injection point at the end of repack_dropped isolation te

Detach injection point at the end of repack_dropped isolation test.

Commit e5d25959cf8 added the repack_dropped isolation test, which
attaches the injection point repack-concurrently-before-lock as a
local one but doesn't detach it. The injection point is then detached
only when the session's backend exits, and isolationtester doesn't
wait for that before starting the next test. If the backend is slow to
exit, the next test that attaches the same injection point, currently
repack_missingval, fails with an "already defined" error.

Fix by detaching it in the session's teardown, as the other REPACK
tests do. Since the session that attached the injection point detaches
it, the exiting backend no longer touches it.

Reported-by: Hayato Kuroda <kuroda(dot)hayato(at)fujitsu(dot)com>
Author: Hayato Kuroda <kuroda(dot)hayato(at)fujitsu(dot)com>
Reviewed-by: Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>
Discussion: https://postgr.es/m/8ccd0497-c914-47e2-804d-299b678863bf@gmail.com
Backpatch-through: 19

Branch
------
REL_19_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/b175cbae02965ab6d60d4526c076ce929ba24933

Modified Files
--------------
src/test/modules/injection_points/expected/repack_dropped.out | 5 +++++
src/test/modules/injection_points/specs/repack_dropped.spec | 5 +++++
2 files changed, 10 insertions(+)