Back-Patch "Add wait_for_subscription_sync for TAP tests."

Back-Patch "Add wait_for_subscription_sync for TAP tests."

This was originally done in commit 0c20dd33db for 16 only, to eliminate
duplicate code and as an infrastructure that makes it easier to write
future tests. However, it has been suggested that it would be good to
back-patch this testing infrastructure to aid future tests in
back-branches.

Backpatch to all supported versions.

Author: Masahiko Sawada
Reviewed by: Amit Kapila, Shi yu
Discussion: https://postgr.es/m/CAD21AoC-fvAkaKHa4t1urupwL8xbAcWRePeETvshvy80f6WV1A@mail.gmail.com
Discussion: https://postgr.es/m/E1oJBIf-0006sw-SA@gemulon.postgresql.org