Add findOneWithAssert to ember-testing

and make click, fillIn, triggerEvent (and hence
keyEvent) make use of it.

This change will cause your test suite to loudly
break if the selectors you provide to the above
helpers match more than one element.

Note: I tried and failed to add test cases. It's very difficult to test a failing assert given QUnit + async, which is likely the same reason that no failing-assertion test cases exist for the original `testWithAssert`.

Also note that `find` is untouched; it's only the above listed helpers that will throw when supplied that helper that matches more than one el..