tor-keymgr: Implemented test improvements

Replace string-based metadata in KeyMgr tests with a structured ItemMetadata type that properly tracks:

Item identifiers (coot, moorhen, etc.)

Generation source keystores

Retrieval source keystores

This change improves test assertions by making them more precise and self-documenting, eliminating the confusing string concatenation pattern previously used for tracking keystore operations.

Fixes #1888 (closed)