Skip tst_QMdiArea::setViewport on macOS due to flakey failures in CI

Task-number: QTBUG-58520
Change-Id: I582c190de45e85e2dfb397289720c655ec8d781c
Reviewed-by: Liang Qi <liang.qi@qt.io>

tests/auto/widgets/widgets/qmdiarea/tst_qmdiarea.cpp

@@ -1507,6 +1507,10 @@ void tst_QMdiArea::setBackground()
 
 void tst_QMdiArea::setViewport()
 {
+#ifdef Q_OS_MACOS
+    QSKIP("Sometimes crashes in the CI, see QTBUG-58520");
+#endif
+
     QMdiArea workspace;
     workspace.show();