Wait for subscription to sync in t/031_column_list.sql

Wait for subscription to sync in t/031_column_list.sql

One of the TAP tests added in 923def9a53 did not wait after creating a
subscription, and wait_for_catchup is not sufficient for this. So if the
tablesync workers happen do not complete quickly enough, the test won't
see the expected results.

This probably explains intermittent failures on a couple buildfarm
animals (komodoensis, petalura and snapper).

Reported-by: Tom Lane
Discussion: https://postgr.es/m/170549.1648330634@sss.pgh.pa.us