General: Add Mock Plugin and fill in gaps in tests

[fubuloubu]

Elevator pitch:

For Testing plugin integration with Core, it would be nice to have a set of "mock plugins" that demonstrated the functionality of core without needing to be fully implementing the plugin API

Value:

Examples:
#318 (comment)

Dependencies:

Design approach:

Task list:

• Tasks go here

Estimated completion date:

Design review:

Do not signoff unless:

• 1. agreed the tasks and design approach will achieve acceptance, and
• 2. the work can be completed by one person within the SLA.
     Design reviewers should consider simpler approaches to achieve goals.

(Please leave a comment to sign off)