Move js-emails-disabled to a better test locator

The following discussion from !98547 (merged) should be addressed:

• @mrincon started a discussion: (+1 comment)

  Suggestion (non-blocking): This was not introduced here but js-emails-disabled should be replaced with a data-testid because it is only used in tests.

  Even then I suspect an improvement that find the checkbox by text label would be better because we "recommend that you query by the element’s text label instead of by ID, class name, or data-testid."