We were not waiting for a publisher to catch up with the subscriber after
creating a subscription. Now, it can happen that apply worker starts
replication even after we have disabled the subscription in the test. This
will make the test expect that there is no active slot whereas there
exists one. Fix this symptom by allowing the publisher to wait for
catching up with the subscription.

It is not a good idea to ensure if the slot is still active by checking
for walsender existence as we release the slot after we clean up the
walsender related memory. Fix that by checking the slot status in
pg_replication_slots.

Also, it is better to avoid repeated enabling/disabling of the
subscription.

Finally, we make autovacuum off for this test to avoid any empty
transaction appearing in the test while consuming changes.

Reported-by: as per buildfarm
Author: Vignesh C
Reviewed-by: Amit Kapila, Michael Paquier
Discussion: https://postgr.es/m/CAA4eK1+uW1UGDHDz-HWMHMen76mKP7NJebOTZN4uwbyMjaYVww@mail.gmail.com