Call _super first in tests.

Due to https://github.com/emberjs/ember.js/commit/f9ecb12e3e21e18e20e1e18abcb10b96683fb242
it not possible to call `this.deferReadiness()` in an Application's
`init` (because this._readinesDeferrals are not greater than 1 which
causes an assertion in the deferReadiness function call).