Use 2x for height and width of #ember-testing.
Use 2x for height and width of #ember-testing.

Prior to this change, the testing preview would have the proper width but only half the height.  

### Before

![screenshot](https://monosnap.com/file/E8UKIAAMmyAjeLBjlrlNIVG8mcGID2.png)

### After

![screenshot](https://monosnap.com/file/GE0c0EpmoPq7IZfjptbdwG3FcncMHN.png)
ember-cli/ember-cli-qunit,ember-cli/ember-cli-qunit