remove duplication of testing for Ember.run.debounce

Debounce's functionality is already being tested in:
https://github.com/ebryn/backburner.js/blob/master/test/tests/debounce_test.js

As per discussion with @stefanpenner, we would have to stub setTimeout
and use a custom scheduler since we can't rely on the browser scheduler
during testing.
Merge pull request #2925 from fivetanley/fix-ember-metal-tests

remove duplication of testing for Ember.run.debounce