Fix test failure caused by commit 76b78721ca.

Fix test failure caused by commit 76b78721ca.

The test failed because it assumed that a newly created logical
replication slot could be synced to the standby by the slotsync worker.
However, the presence of an existing physical slot caused the new logical
slot to use a non-latest xmin. On the standby, the DDL had already been
replayed, advancing xmin, which led to the slotsync worker failing to sync
the lagging logical slot.

To resolve this, we moved the slot sync statistics tests to run after the
tests that do not require the newly created slot to be sync-ready.

As per buildfarm.

Author: Hou Zhijie <[email protected]>
Reviewed-by: Hayato Kuroda <[email protected]>
Reviewed-by: Amit Kapila <[email protected]>
Discussion: https://postgr.es/m/OSCPR01MB14966FE0BFB6C212298BFFEDEF5D1A@OSCPR01MB14966.jpnprd01.prod.outlook.com