TW Test Suite Should be Improved

[pmario]

Parenthetically, this appears to be another of those cases where our test suite is inadequate. We’ve talked elsewhere about this, but it really seems that fixing our test suite should be our highest priority task at the moment.

Originally posted by @Jermolene in #9695 (comment)

[pmario]

@Jermolene ... Can you be a bit more specific about this one.

[Jermolene]

Hi @pmario I was thinking particularly of the various v5.4.0 regressions that we've encountered. The existing testing frameworks are adequate for JS and wikitext, but don't allow for even basic UI testing. This is a good time to tackle this because (a) LLMs can help us measure and improve test coverage and (b) better tests will make it easier for LLMs to improve or extend TiddlyWiki.