Fix duplicate IPSLA counts in post-upgrade callback check

[monrog2]

Duplicate IPSLA objects with the same DN can make Post Upgrade Callback Integrity report an outage even when the old and converted policy counts match. For example, issue #344 reports a count of two fvSlaDef objects while the listing contains one policy.

Query fvSlaDef and fvIPSLAMonitoringPol directly and compare their unique DN counts after all pages are retrieved. This also handles duplicates in plain class responses. Genuine count mismatches still produce FAIL - OUTAGE WARNING!!; the other callback checks, version gates, and query error handling retain their existing behavior.

Add synthetic fixtures and regressions for duplicates in either class, duplicates across pages, missing objects masked by duplicates, extra objects, empty classes, same-name policies in different tenants, malformed objects, API errors, and incomplete responses.

Validation:

• Focused callback and icurl tests: 39 passed.
• Full unit suite on Python 3.9.6: 1,433 passed.
• Python 3.8 syntax parsing and git diff --check: passed.
• Python 2.7 and 3.8 runtime testing remains for CI; those interpreters are unavailable locally.

Related defect: CSCwt30743.

Fixes #344.