Support test isolation validation out of the box.

Test isolation validation is currently available in `ember-qunit@4.1.2`. This provides debugging information to users when their tests have async behavior that continues after the test is 'done'.

To support this as a default feature in ember-qunit, we should ensure we enable backburner's `DEBUG` flag by default, which allows backburner to capture stacks at the callsite of runloop work. Having the extended debugging information is necessary to provide the full information to `ember-qunit` for test debugging.