On FreeBSD, the new test fails due to a WAL file being removed before
the standby has had the chance to copy it.  Fix by adding a replication
slot to prevent the removal until after the standby has connected.

Author: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Reported-by: Matthias van de Meent <boekewurm+postgres@gmail.com>
Discussion: https://postgr.es/m/CAEze2Wj5nau_qpjbwihvmXLfkAWOZ5TKdbnqOc6nKSiRJEoPyQ@mail.gmail.com