Fix a test in failover slots regression test.

Fix a test in failover slots regression test.

Wait for the standby to catch up before syncing the slots with
pg_sync_replication_slots(), otherwise, the logical slot could be ahead
and the sync would fail.

The other way to fix the test is to change it to use slotsync worker and
poll for the sync to get finished but the current approach is better as
this is a predictable way to write the test.

Per buildfarm

Author: Hou Zhijie
Reviewed-by: Amit Kapila
Discussion: https://postgr.es/m/OS0PR01MB571665359F2F5DCD3ADABC9F94002@OS0PR01MB5716.jpnprd01.prod.outlook.com