This integration test fails intermittently because the undo button can
only be activated if focus can be put on it, and that in turn can only
happen if it's visible. The test tried to make sure that the undo bar
is visible, but checking for the absence of the `hidden` attribute is
unfortunately not enough to assert visibility according to Puppeteer
documentation [1]. Moreover, the undo button wasn't checked at all.
To fix the issue we let Puppeteer do the visibility detection for the
undo bar by providing the `visible: true` option to `waitForSelector`
[2]. This is consistent with the other tests that already do this, and
also with the existing code that detects if the undo bar is hidden
(which uses the `hidden: true` option of `waitForSelector`). Moreover,
we wait for the undo button to be present before putting focus on it.
For consistency, and to avoid intermittent failures elsewhere, we mirror
this solution to the other undo bar/button tests of the various editors.
[1] https://pptr.dev/api/puppeteer.elementhandle.isvisible
[2] https://pptr.dev/api/puppeteer.waitforselectoroptions
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