test method accepts an optional `expected` argument

QUnit.test can be called with the number of expectations (see https://github.com/jquery/qunit/blob/v1.14.0/qunit/qunit.js#L111).
This updates ember-qunit to allow this, also.

I added a test with this syntax. It would be better to have a test that shows that the expectation is honored (i.e., show that a test with 2 expectations but declared as `test('some name', 1, function()...)` would fail).