testlib: use const methods more

to avoid implicit detach

Change-Id: I8c7de3b7d6a84299714f837da5c9feb39074d0c2
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/testlib/qtestaccessible.h

@@ -108,7 +108,7 @@ public:
             qWarning("Timeout waiting for accessibility event.");
             return false;
         }
-        const bool res = *eventList().first() == *ev;
+        const bool res = *eventList().constFirst() == *ev;
         if (!res)
             qWarning("%s", qPrintable(msgAccessibilityEventListMismatch(eventList(), ev)));
         delete eventList().takeFirst();