Test that attached Keys behavior is identical to TextInput/Area

Such composite controls that consist of a TextInput or TextArea should
offer identical behavior for the Keys attached property on the control
itself as it would behave if it was used on the internal editor.

Task-number: QTBUG-38289
Change-Id: I4e6d77d2cc1f3c7202418b9aa21d1f40ddd0eb96
Reviewed-by: Liang Qi <liang.qi@digia.com>