Inappropriate use of QSKIP in QMdiSubwindow autotest

In qtbase/tests/auto/widgets/widgets/qmdisubwindow/tst_qmdisubwindow.cpp, several test functions are effectively disabled with:

QSKIP("Until we have a QEvent::WindowFlagsChange event, this will skip");

These QSKIPs would be better written as QEXPECT_FAIL, so that if the feature appears the test will report that it needs to be updated. Regardless of whether this is a QSKIP or a QEXPECT_FAIL, the message must refer to a bug report that requests the needed feature.