Fix the race condition in the test added by 7c99dc587.

Fix the race condition in the test added by 7c99dc587.

After executing ALTER SUBSCRIPTION tap_sub SET PUBLICATION, we did not
wait for the new walsender process to restart. As a result, an INSERT
executed immediately after the ALTER could be decoded and skipped,
considering it is not part of any subscribed publication. And, the old
apply worker could also confirm the LSN of such an INSERT. This could
cause the replication to resume from a point after the INSERT. In such
cases, we miss the expected warning about the missing publication.

To fix this, ensure the walsender has restarted before continuing after
ALTER SUBSCRIPTION.

Reported-by: Tom Lane as per CI
Author: vignesh C <vignesh21@gmail.com>
Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Discussion: https://postgr.es/m/1230066.1745992333@sss.pgh.pa.us