Fix focusin-based tests

[Telroshan]

Description

Even though we call .focus on elements in these 2 tests, the focusin event might not be fired.

How to reproduce it?

Disclaimer: the CI suddenly constantly fails on this, and I have no idea why, specifically why it worked until now and suddenly doesn't anymore.
To reproduce that issue, run the tests locally through npx serve (thus browser context) with a 20x CPU slowdown throttling. Refresh the tests page, and click outside it before these tests run.
Adding logs reveal that, even though the document.activeElement gets properly updated to the focused element, the focusin event won't fire, until the window gets focused back (clicking inside it).
I guess that's an expected browser-UX feature, where the browser waits for the user to actually focus the element (thus focus the browser window first) before notifying it.
The issue here is that we don't really know how things work when running headless through mocha-chrome, but I'm thinking this could explain why the pipeline fails.

Suggested solution

The suggested solution here simply doubles the focusin event by manually triggering it with htmx.trigger.
This doesn't change the test as, what we're testing is not the focusin event itself being fired (that's the browser's job), just ensuring that htmx behaves correctly once that event has been fired.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded

[alexpetros]

Yup, totally reasonable