Enhance test suite around zola serve

[jamwil]

Setting this up to discuss/track the addition of more thorough endpoint testing around the serve command. Better test coverage will support some refactoring efforts, including #2446 along with a better url path mounting implementation as discussed in #2311.

[jamwil]

Apologies for the lack of action on this so far. It's still on my radar—energy level be damned.

[jamwil]

Just marking here that the unit tests introduced in #2484 seem to fail intermittently in Windows CI. Will leave this issue and try to identify what's causing that.

[jamwil]

Hmm. So I finally got around to testing these on a Windows box. All green. Unfortunately, this is CI-specific.

Edit: One thing I can try is to copy the test site into a temporary directory on each test invocation. That should, in theory, fix any concurrency issue.