Testing/Debugging Refinements

[noelforte]

This pull tracks refinements to tests for this plugin, notably addressing further issues with caching templates.

Goals:

• Move all basic functionality tests to /tests/stubs/
• Refactor test files that cover the above functionality into a single test suite, keep serve, autotrim, ignore-tag and other custom features seperate for erganomics
• Add dependency on debug package (perhaps as a peer, to pull in from eleventy) (added in b8f0a03)

[noelforte]

Decided to forgo testing serve mode since it's a bit involved to set up and it's simpler to rely instead on emperical observations. If there are more issues with caching then I can add test cases to handle it.