Call waitForExplicitFinish in test_img_width_height.html.

This fixes an intermittent failure introduced in
<https://github.com/servo/servo/pull/3929>.

tests/content/test_img_width_height.html

@@ -7,6 +7,7 @@
   <img src="test.png"/>
 <script>
 // Testing get/set of image width/height properties
+waitForExplicitFinish();
 
 var img = window.document.getElementsByTagName("img")[0];