Bump ember-a11y-testing from 2.0.0 to 3.0.0

Bumps [ember-a11y-testing](https://github.com/ember-a11y/ember-a11y-testing) from 2.0.0 to 3.0.0.
- [Release notes](https://github.com/ember-a11y/ember-a11y-testing/releases)
- [Changelog](https://github.com/ember-a11y/ember-a11y-testing/blob/master/CHANGELOG.md)
- [Commits](ember-a11y/ember-a11y-testing@v2.0.0...v3.0.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Author: dependabot-preview[bot]