Add more tests

[WebCoder49]

Definitely

• Into the tests folder, improve the tests by adding more for forms, and checking there is good test coverage.

Maybe

• Add tests for bugs found in issues that might recur in future versions.
• Add more unit tests / use an actual test framework with live coverage.
• Check test coverage / apply tests in PRs.

[WebCoder49]

Add more for changing template attribute value, programmatically creating code-input elements (done), etc. Support feels patchy.

[WebCoder49]

Potentially use https://www.selenium.dev/ or similar, rather than personally hacked-together code.

[WebCoder49]

Add more for changing template attribute value

Especially once #227 and #226 are fixed, to prevent further regressions.