Mango unit tests

[jcoglan]

Overview

As requested in #5858 (comment), this PR just adds a lot of unit tests for Mango selector matching, to make it easier to extend and refactor this code in future.

Related Issues or Pull Requests

• Mango match_failures/2 function #5858

Checklist

• This is my own work, I did not use AI, LLM's or similar technology
• Code is written and works correctly
• Changes are covered by tests
• Any new configurable parameters are documented in rel/overlay/etc/default.ini
• Documentation changes were made in the src/docs folder
• Documentation changes were backported (separated PR) to affected branches