Blacklist tst_QWebEnginePage::runJavaScriptFromSlot on macOS

This is our most flaky test at the moment

Task-number: QTBUG-74718
Change-Id: Ia7668347a74982abfb0297169f564db89944e87c
Reviewed-by: Peter Varga <pvarga@inf.u-szeged.hu>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

tests/auto/widgets/qwebenginepage/BLACKLIST

@@ -12,3 +12,6 @@ windows
 
 [getUserMediaRequestDesktopVideoManyRequests]
 windows
+
+[runJavaScriptFromSlot]
+osx