Add wait_for_subscription_sync for TAP tests.

Add wait_for_subscription_sync for TAP tests.

The TAP tests for logical replication in src/test/subscription are using
the following code in many places to make sure that the subscription is
synchronized with the publisher:

  $node_publisher->wait_for_catchup('tap_sub');
  $node_subscriber->poll_query_until('postgres',
    qq[SELECT count(1) = 0
       FROM pg_subscription_rel
       WHERE srsubstate NOT IN ('r', 's')]);

The new function wait_for_subscription_sync() can be used to replace the
above code. This eliminates duplicated code and makes it easier to write
future tests.

Author: Masahiko Sawada
Reviewed by: Amit Kapila, Shi yu
Discussion: https://postgr.es/m/CAD21AoC-fvAkaKHa4t1urupwL8xbAcWRePeETvshvy80f6WV1A@mail.gmail.com