Add `.toBeAccessible()`

[Simon Abbott]

Describe the feature you'd like:

testing-library exposes the isInaccessible function, but using it to test for accessibility leads to a less than ideal testing experience. It would be nice if there was a first-class .toBeAccessible() expectation we could use to check for accessible components.

Suggested implementation:

A simple wrapper around isInaccessible, but with an actually useful error message.

Describe alternatives you've considered:

Alternative 1:

expect(isInaccessible(element)).toBe(false);

Pros: Works without modifying @testing-library/jest-dom

Cons:

• Bad error messages ("expected true to be false")
• Unclear intent due to double negative ("wait, do we want it to be inaccessible?")
• Unnecessarily verbose

Alternative 2:

Write a custom matcher.

Pros: Works without modifying @testing-library/jest-dom

Cons: Hard to make it work cleanly without needing to re-invent functions such as checkHtmlElement

Teachability, Documentation, Adoption, Migration Strategy:

Due to the simple nature of this matcher the docs should only need the bare minimum of an example and a link to the isInaccessible docs.