Fix flaky aria-disabled tests

[josiasds]

@chawes13 found a flaky test when reviewing the RTL Cheat Sheet.

We were only checking for the existence of the aria-disabled attribute but not its value. If aria-disabled were false, the test would still pass, and that's not the desired outcome.

Release Notes

Author Checklist

• Add unit test(s)
• Update version in package.json (see the versioning guidelines)
• Update documentation (if necessary)
• Add story to storybook (if necessary)
• Assign dev reviewer

[chawes13]

I noticed this too! You beat me to it. Thanks a bunch 🙏