pgsql: Fix recently added incorrect assertion

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8328a15f8f95cad3fa99bbab551cade0a8403708

Modified Files
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src/backend/access/transam/xlog.c         |  5 ++---
src/test/recovery/t/020_archive_status.pl | 14 ++++++++++++++
2 files changed, 16 insertions(+), 3 deletions(-)