tst_qquickmenu: fix another "cannot show menu: parent is null" warning

The test shows it after the menu gets a window anyway, so we don't need
to try to make it immediately visible.

Same fix as 1025a40812.

Amends d0a83e3330.

Pick-to: 6.5 6.8 6.9 6.10
Change-Id: I7cfd59ae88b3ce4ab9b4ec780838d7cdc476091d
Reviewed-by: Santhosh Kumar <santhosh.kumar.selvaraj@qt.io>

tests/auto/quickcontrols/qquickmenu/data/scrollableWithFixedHeight.qml

@@ -15,7 +15,7 @@ Window {
         id: menu
         anchors.centerIn: parent
         height: 100
-        visible: true
+
         Repeater {
             model: 10
             delegate: MenuItem {