Popup: try to stabilize mousePropagation test

Wait for the popup to be opened before simulating the click.

Change-Id: I3f7b64ecbdd19981e6b74ed03a732c279b0282cf
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>
(cherry picked from commit 40f448ad16)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit e81abf9dea)

tests/auto/quickcontrols/controls/data/tst_popup.qml

@@ -1475,6 +1475,8 @@ TestCase {
 
         let popup = window.popup
         popup.open()
+        tryCompare(popup, "opened", true)
+        waitForRendering(popup.contentItem)
 
         // mouse clicks into the popup must not propagate to the parent
         mouseClick(window)