Cover lib/ and src/ with equivalent and extensive unit tests

[ralflang]

• PHPUnit 11-13 coverage
• Might reveal issues
• Cover both "configure" and "inheritance" use cases.
• PHP 8.1 - 8.5 target. If it runs on earlier platforms, good.

[ralflang]

Made some progress recently.

[ralflang]

Changed target to 8.1+ as I don't see us relying on anything genuinely requiring 8.2 here