Addon A11y: Toggle default focus of "tests completed" button

[Nevnet99]

Closes #24129

What I did

Adds a boolean to the component that allows you to revert specific buttons to have the default focus for the button (previously it had the bottom focus rather than all around).

Manual testing

This PR does not have a canary release associated. You can request a canary release of this pull request by mentioning the @storybookjs/core team here.

core team members can create a canary release here or locally with gh workflow run --repo storybookjs/storybook canary-release-pr.yml --field pr=<PR_NUMBER>

[yannbf]

Hey @Nevnet99 thanks for contributing to Storybook!

There are a couple of things that are worth mentioning:

1. What you're trying to fix might not be a bug. Please read this comment.
2. It seems that this change introduced a side-effect in every addon panel that uses the button. Now they are always showing as if they're focused. The proposed fix doesn't seem to be the correct solution, as the original issue relates to keyboard focus being lost, while the contribution only makes visual changes to the element shown in the addon panel.

I think the issue needs a little more discussion, feel free to be part of it!