Add (proper) tests

[robinmetral]

There's already a light testing setup for this project, that checks that the project website (very basic) renders.

With complexity increasing, I'd like to improve this setup and add more cases. I'm thinking of a new tests package in the monorepo (so actually separate from the project website at www), where each subdir would be a mini brut website with different test cases.

Some things to cover:

• site using a collection (default, under /pagesDir/collectionDir)
• site using several collections
• site using a collection at /pagesDir/some/path/collectionDir
• site using a collection with subdirs: /pagesDir/collectionDir/2024/post.md and /pagesDir/collectionDir/2023/post.md should be in the same collection
• site using a custom processContext function
• site using a custom buildScript

...and more, add to the list above ☝️

• I'd also like to add coverage to have better visibility on what's covered and not (not necessarily aiming for 100%)