E2E test changes for #64952 - Prevent empty Quick Draft submissions from WordPress dashboard

[audrasjb]

https://core.trac.wordpress.org/ticket/64952

This copies changes from #11380 and add E2E tests changes.

[github-actions]

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Props audrasjb, joedolson.

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[github-actions]

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[audrasjb]

Looks like the E2E tests are fixed now. I think the implementation is ready for review.
I'm closing the other PR in favor of this one.

Note for the future committer: don't forget to add the props from the first PR: opurockey, westonruter

[joedolson]

Should also add a parallel test verifying that a draft can be created with title but without content. Basically the same thing, just using title content.

[audrasjb]

Right @joedolson 👍 New test added for post with Title but without Content.

[joedolson]

The setCustomValidity method doesn't voice errors for screen readers, so while the functionality is good, the error message will need some additional accessibility enhancements.

I think it would probably be better, and more consistent with other experiences in the admin, to use an inline admin notice and wp.a11y.speak() to make the error both more visually integrated and accessible.

[audrasjb]

Thanks for the feedback!