Improve testing setup.

[okamsn]

• Don't test the compilation of files for which the Emacs version is too low for the UI. For example, Vertico requires Emacs 27.1, so we shouldn't bother testing vertico-prescient.el with Emacs versions less than that. This makes implementations a bit simpler.
• To do that, explicitly pass the version argument for the docker make target.
• Don't raise an error for long line of package requirements.

[okamsn]

✨ Looks fine, although I do have a comment at #156 (comment). I looked at the actual runtime and I see building the Docker image only takes 15 seconds, so I guess it doesn't matter too much. Is the motivation for splitting the make targets out into a matrix to report easier to read results on the GitHub status checks?

Yes, but I realized that I was focusing on the wrong thing when I wrote this. For #156, it only needs that the "Package Require" line can be longer than 80 characters. I meant to make a new PR for just that, but accidentally pushed that change to the main branch instead of creating a PR. My apologies.

[raxod502]

No worries!