tst_qmessagebox was disabled because it crashed.
It does not any more. Now it hangs, which
is a new problem.
Fixes: QTBUG-87671
Task-number: QTBUG-101217
Pick-to: 6.2 6.3
Change-Id: Iec15cf82a4112ee79182ec091622e40147bd3e7f
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>