Fix compiler warning about discarded return value

Not testing the result of the copying, just verifying that it doesn't
assert.

Pick-to: 6.0
Change-Id: Ib51129134b74c17eada7e3819ccfff10bb9affbf
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

tests/auto/gui/image/qimage/tst_qimage.cpp

@@ -1220,7 +1220,7 @@ void tst_QImage::copy()
     // Task 99250
     {
         QImage img(16,16,QImage::Format_ARGB32);
-        img.copy(QRect(1000,1,1,1));
+        (void)img.copy(QRect(1000,1,1,1));
     }
 }