tst_qmessagebox is flaky

FAIL! : tst_QMessageBox::defaultButton() Compared pointers are not the same
Loc: [tst_qmessagebox.cpp(250)]

The test tries to verify that a Qt::Key_Enter keyevent clicks the defaultButton, but the test result is that it clicks cancel instead. In the test, there's a previous event sent that cancels the dialog, so this looks like an occasional triggering of the cancel event twice. It certainly looks like this might be a real bug.