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

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