Add hasDescription assertion that works with aria-describedby

[EvgenyOrekhov]

Similar to https://github.com/testing-library/jest-dom#tohavedescription.

I'm trying to use Testing Library with QUnit, and I think hasDescription() would fit really nice.

Example:

<label for="email">Email</label>
<input
  id="email"
  type="email"
  aria-describedby="emailMessage"
/>
<small id="emailMessage">Email is invalid</small>

assert.dom(getByLabelText("Email")).hasDescription("Email is invalid");