Fix Subscription test added by commit ce0fdbfe97.

 Fix Subscription test added by commit ce0fdbfe97.
 
 We want to test the variants of Alter Subscription that are not allowed in
 the transaction block but for that, we don't need to create a subscription
 that tries to connect to the publisher. As such, there is no problem with
 this test but it is good to allow such tests to run with
 wal_level = minimal and max_wal_senders = 0 so as to keep them consistent
 with other tests.
 
 Reported by buildfarm.
 
 Author: Amit Kapila
 Reviewed-by: Ajin Cherian
 Discussion: https://postgr.es/m/CAA4eK1Lw0V+e1JPGHDq=+hVACv=14H8sR+2eJ1k3PEgwKmU-jQ@mail.gmail.com