[META] Tests [#3432275]

This meta will keep track of many test related issues (many of which need to be created atm)

JsxAjaxInGroupTest is ported from Core's AjaxInGroupTest. This test does not currently work as it doesn't seem like the AJAX replaceWith for the use case in that test. There should be an issue for addressing this, which will require getting replaceWith to work and may require refactoring the test a bit too
The Functional Javascript tests are largely ones ported from Core's AJAX tests, and other than AjaxInGroupTest, they all (if memory serves) pass locally. However, these same tests fail intermittently on Gitlab CI, largely due to timing issues that result in Elements not found or stale elements not being part of the DOM. These tests are still in the filesystem, but many use markTestSkipped() since they do not reliably pass on Gitlab CI. There are also many arbitrary ->wait() uses that should be replaced with something that waits for something specific to occur and not just the current arbitrary pauses.
The tests and or support code that runs alongs tests need to be updated so we don't markTestSkipped() and wait() uses should eventually be removed
We should look into tests that compare default Umami output to the output of Umami JSX. There will clearly be some differences that are expected and fine - but more parts of a page the test compares, the more effective the test can be in detecting changes that break something.

[META] Tests