Stabilize tst_qquickmousearea.

Replace createView() by initView(), instantiate windows on stack or
use QScopedPointer. Center view in available geometry and make
sure the mouse cursor is outside, since otherwise it can
interfere with drag->active(). Remove insignification
introduced by c1fbee270f50c44f47d4eb287765ad60f362c87a .

Task-number: QTBUG-33006
Change-Id: I8fc661444bc42899dfa3d87887c594270483e508
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>