Fix cfbot failure caused by commit 228c370868.

Fix cfbot failure caused by commit 228c370868.

Ensure the test waits for the apply worker to exit after disabling the
subscription. This is necessary to safely enable the retain_dead_tuples
option. Also added a similar wait in another part of the test to prevent
unintended apply worker activity that could lead to test failures
post-subscription disable.

Reported by Michael Paquier as per cfbot.

Author: Zhijie Hou <houzj.fnst@fujitsu.com>
Discussion: http://postgr.es/m/aIGLgfRJIBwExoPj@paquier.xyz