Repair uncovered warning in persistnce.py; update versioning tests

Repair uncovered warning in persistnce.py; update versioning tests

Repaired a warning in the ORM flush process that was not covered by  test
coverage when deleting objects that use the "version_id" feature. This
warning is generally unreachable unless using a dialect that sets the
"supports_sane_rowcount" flag to False, which  is not typically the case
however is possible for some MySQL configurations as well as older Firebird
drivers, and likely some third party dialects.

As part of this change, we would like to establish that versionining-
related warnings are not expected to emit for the core set of
test-covered dialects, and if they are we would like the check for
warning emitted to be explicit, rather than an ignore.

Fixes: #5068
Change-Id: Iee097554e43cbb54ced1be056635809e67cf30d6