tst_spinbox: destroy dyn. created spinbox before leaving test

Change-Id: Ifceb9c69e94d3c4fd31169785fb6a5b8c8694561
Reviewed-by: J-P Nurmi <jpnurmi@theqtcompany.com>

tests/auto/controls/data/tst_spinbox.qml

@@ -157,6 +157,7 @@ Item {
                 keyClick(data.input[i])
 
             compare(spinbox.value, data.value)
+            spinbox.destroy()
         }
 
         function test_increment_mouse() {