Fix race condition in subscription TAP test 021_twophase

Fix race condition in subscription TAP test 021_twophase

The test did not wait for all the subscriptions to have caught up when
dropping the subscription "tab_copy".  In a slow environment, it could
be possible for the replay of the COMMIT PREPARED transaction "mygid"
to not be confirmed yet, causing one prepared transaction to be left
around before moving to the next steps of the test.

One failure noticed is a transaction found in pg_prepared_xacts for the
cases where copy_data = false and two_phase = true, but there should be
none after dropping the subscription.

As an extra safety measure, a check is added before dropping the
subscription, scanning pg_prepared_xacts to make sure that no prepared
transactions are left once both subscriptions have caught up.

Issue introduced by a8fd13cab0ba, fixing a problem similar to
eaf5321c3524.

Per buildfarm member kestrel.

Author: Vignesh C <vignesh21@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Discussion: https://postgr.es/m/CALDaNm329QaZ+bwU--bW6GjbNSZ8-38cDE8QWofafub7NV67oA@mail.gmail.com
Backpatch-through: 15