Fix test failure in 021_twophase.pl.

The test is expecting two prepared transactions corresponding to two
subscriptions but it waits to catch up for just one subscription. Fix it
by allowing to wait for both subscriptions.

Reported-by: Michael Paquier, as per buildfarm
Author: Ajin Cherian
Reviewed-By: Amit Kapila, Vignesh C, Peter Smith
Discussion: https://postgr.es/m/CAA4eK1+_0iNQ8Z=KVTjmmAqNX-hyv+1+fnZ-Yx8CVP=uAcekqw@mail.gmail.com

Author: Amit Kapila

src/test/subscription/t/021_twophase.pl

@@ -316,7 +316,9 @@
     INSERT INTO tab_copy VALUES (99);
     PREPARE TRANSACTION 'mygid';");
 
+# Wait for both subscribers to catchup
 $node_publisher->wait_for_catchup($appname_copy);
+$node_publisher->wait_for_catchup($appname);
 
 # Check that the transaction has been prepared on the subscriber, there will be 2
 # prepared transactions for the 2 subscriptions.