Hi, I dont know if Your problem is exists yet or not, I found this solution:

Qt Code: 
  1. void MyTest::testPasswordDialog()
  2. {
  3.     QTimer::singleShot(500, this, SLOT(TimeOut()));
  4.     QVERIFY(functionThatProducesMessageBox());
  5. }
  6.  
  7. void MyTest::TimeOut()
  8. {
  9.     QWidgetList allToplevelWidgets = QApplication::topLevelWidgets();
  10.     foreach (QWidget *w, allToplevelWidgets) {
  11.         if (w->inherits("QMessageBox")) {
  12.             QMessageBox *mb = qobject_cast<QMessageBox *>(w);
  13.             QTest::keyClick(mb, Qt::Key_Enter);
  14.         }
  15.     }
  16. }
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