Make global ARIA attributes configurable in aria-allowed-attr

[WilcoFiers]

To make axe-core more flexible and allow it to support different environments it would be helpful to add more configuration options. One such option is to be able to set which global attributes are available in aria-allowed-attr, see #2078 for details.

[straker]

QA: to test this you can take a rule such as aria-allowed-attrs and test any role in the new standards object that has an allowedAttrs property. First test that you can't add an aria-attribute that is not one 1) global 2) part of the reuqiredAttrs and 3) part of the allowedAttrs, then configure the role to allow the aria-attribute using axe.configure.

For example, the checkbox role allows attributes aria-checked, aria-readonly, and aria-required. To test, you can add aria-expanded to an element that has role=checkbox and see that it should fail aria-allowed-attrs rule. Then configure the standards object to allow it and test it again, and it should pass.

<div role="checkbox" aria-expanded="true" aria-checked="true"></div>

<script>
axe.configure({
  standards: {
    // the standards object to configure https://github.com/dequelabs/axe-core/blob/develop/lib/standards
    ariaRoles: {
      // the role to configure
      checkbox: {
        // the property to configure. with arrays you'll need to copy what's in the standards
        //  object and add the new values, otherwise the previous values will be removed
        // (meaning `aria-checked`, etc. would no longer be allowed)
        allowedAttrs: ['aria-checked', 'aria-readonly', 'aria-required', 'aria-expanded']
      }
    }
  }
});
</script>

await axe.run();  // so not show violation for `aria-allowed-attrs` for `aria-expanded` on the checkbox role

You should also test other rules in a similar manner such as aria-valid-attr (by adding a new attribute to the ariaAttrs object), aria-valid-attr-value, aria-roles (by adding a new role to the ariaRoles object), aria-required-parent (requiredContext property), and aria-required-children (requiredOwned property).

[padmavemulapati]

Verified as per QA notes using latest axe-core(3.5.5 pulled code)
Step1: for the test file <div role="checkbox" aria-expanded="true" aria-checked="true">Hello</div>
Simply running axe - would result - failure for aria-allowed-attr (attached the Screenshot)

Step2: configured with axe.configure({ standards: { ariaRoles: { checkbox: { allowedAttrs: ['aria-checked', 'aria-readonly', 'aria-required', 'aria-expanded'] } } } });
and did - axe.run(); - resulting - pass for aria-allowed-attr (attached the screenshot)

[jeankaplansky]

Included in internal release notes 7/27/2020