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