Stabilize regression test and catching-up to the core.

Stabilize regression test and catching-up to the core.

Further unstability is found about stats reset. Inserted more 1 second
sleeps to stabilize it. Some planner fix (maybe 72a626e688) seems to
have changed its behvavior. The commit 041ad9a66d changes the result
of A-8-4-10 in ut-A. The commit 88275ac199, which changed the category
of client_min_messages, affected ut-A's result. Follow the changes.

Most of this fix is comes from the pull request
https://github.com/ossc-db/pg_hint_plan/pull/28 by Dagouhan (smoon
<30999182+Dagouhan@users.noreply.github.com>).