Make tst_dialogs less flaky

Wait to finish javascript execution before opening
next dialog.

Task-number: QTBUG-72113
Change-Id: I383e3fc3c405a94ddcbd74fe1cc7ab0d81c4e315
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

tests/auto/quick/dialogs/tst_dialogs.cpp

@@ -218,6 +218,7 @@ void tst_Dialogs::javaScriptDialogRequested()
     QCOMPARE(dialog->type(), type);
     QCOMPARE(dialog->message(), message);
     QCOMPARE(dialog->defaultText(), defaultText);
+    QTRY_VERIFY(m_listner->ready()); // make sure javascript executes no longer
 }
 
 #include "tst_dialogs.moc"