Fix tst_qquicktextinput on Windows and Mac.

Both require a sensible window size.

Task-number: QTBUG-23485
Task-number: QTBUG-28611
Change-Id: If888dd4d60fccdabc9856f914d6542ced6a41d64
Reviewed-by: Janne Anttila <janne.anttila@digia.com>
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@digia.com>
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>
Reviewed-by: Christopher Adams <chris.adams@jollamobile.com>