Fix recently added incorrect assertion

Alvaro Herrera <alvherre@alvh.no-ip.org>

Commit: 8328a15f8f95cad3fa99bbab551cade0a8403708
Author: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2022-10-24T10:02:33Z
Releases: 16.0
Fix recently added incorrect assertion

Commit df3737a651f4 added an incorrect assertion about the preconditions
for invoking the backup cleanup callback: it misfires at session end in
case a backup completes successfully.  Fix it, using coding from Michaël
Paquier.  Also add some tests for the various cases.

Reported by Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/20221021.161038.1277961198945653224.horikyota.ntt@gmail.com

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