Fix position of Ember testing root element.
Fix position of Ember testing root element.

The CSS provided by mocha differs from qunit's so absolute
positioning doesn't work.

cowboyd/ember-cli-mocha,blimmer/ember-cli-mocha,HeroicEric/ember-cli-mocha,switchfly/ember-cli-mocha,trentmwillis/ember-cli-mocha,switchfly/ember-cli-mocha,zenefits/ember-cli-mocha,HeroicEric/ember-cli-mocha,trentmwillis/ember-cli-mocha,lazybensch/ember-cli-mocha,ember-cli/ember-cli-mocha,zenefits/ember-cli-mocha,cowboyd/ember-cli-mocha,lazybensch/ember-cli-mocha,blimmer/ember-cli-mocha,ember-cli/ember-cli-mocha