Fix flaky test of data-wp-on-window directive

[luisherranz]

What?

Another attempt to fix #58398.

Why?

It keeps failing sometimes.

How?

I added this line to expect that the event listener is attached, but didn't use expect to make sure Playwright waits until this happens. Instead, I used waitForSelector:

// Make sure the event listener is attached.
await page.waitForSelector('[data-testid="isEventAttached"]:has-text("yes")');

I also removed the other test because it was unnecessary. The longer test was covering the same again.

[github-actions]

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Props: luisherranz, mamaduka.

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Co-authored-by: luisherranz <luisherranz@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>

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[github-actions]

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