Fix tst_qquicktextedit::cursorRectangle_QTBUG_38947()

Fix the uninitialize variable and make sure the test creates
a window that has a sensible (200x200) size.

Change-Id: I02616ab3c832276921e84ae98b7ed926d8fc5f5e
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Liang Qi <liang.qi@digia.com>