Fix the race condition in the test case added by commit a6e04b1d20.

Fix the race condition in the test case added by commit a6e04b1d20.

The commit a6e04b1d20 added a test to ensure that the invalidated logical
slots don't retain WAL. The test was ensuring that the checkpoint removes
the WAL files corresponding to invalidated logical slots on the standby
node but missed the point that the standby node also had a physical slot
which led to the prevention of WAL file removal. Move the creation of
physical slot on the standby and initialization of cascading standby closer
to the test case that actually required it so that other tests don't get
affected by the presence of the physical slot on standby.

Author: Bertrand Drouvot
Reviewed-by: Amit Kapila
Discussion: https://postgr.es/m/2fefa454-5a70-2174-ddbf-4a0e41537139@gmail.com