Make new replication slot test code even less racy
Further fix the test code in
ead9e51e8236, this time by waiting until
the checkpoint has completed before moving on; this ensures that the
WAL segment removal has already happened when we create the next slot.
Author: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/
20210719.111318.
2042379313472032754.horikyota.ntt@gmail.com