Update to new async syntax

[Anthony Trama]

Examples in the docs like

import a11yAudit from 'ember-a11y-testing/test-support/audit';

// ...elided for brevity

test('Some test case', function(assert) {
  visit('/');
  a11yAudit();
  andThen(() => assert.ok(true, 'no a11y errors found!'));
});

should be updated to:

import a11yAudit from 'ember-a11y-testing/test-support/audit';

// ...elided for brevity

test('Some test case', async function(assert) {
  visit('/');
  await a11yAudit();
  assert.ok(true, 'no a11y errors found!');
});

...to follow the new async syntax. And maybe add a note how to use this addon for anyone still using the 2x branch. If this change is okay, I can create a PR with the changes

[Trent Willis]

👍 sounds good to me!