Stablize tests added in 3abe9dc188.

Stablize tests added in 3abe9dc188.

The problem is that after the ALTER SUBSCRIPTION tap_sub SET PUBLICATION
command, we didn't wait for the new walsender to start on the publisher.
Immediately after ALTER, we performed Insert and expected it to replicate.
However, the replication could start from a point after the INSERT location,
and as the subscription isn't copying initial data, we could miss such an
Insert.

The fix is to wait for connection to be established between publisher and
subscriber before starting DML operations that are expected to replicate.

As per CI.

Reported-by: Andres Freund <andres@anarazel.de>
Author: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Discussion: https://postgr.es/m/CALDaNm2ms1deM5EYNLFEfESv_Kw=Y4AiTB0LP=qGS-UpFwGbPg@mail.gmail.com