Fixing test that broke in FF with ff1f99a03f.

tests/Layer/Grid.html

@@ -1375,7 +1375,8 @@
 
         map.zoomTo(1);
 
-        t.delay_call(1, function() {
+        // Mark one tile loaded, to see if back buffer removal gets scheduled.
+        layer.grid[1][1].onImageLoad();
 
         t.ok(layer.backBuffer.parentNode === layer.div,
              '[a] back buffer is a child of layer div');
@@ -1396,7 +1397,6 @@
         // tear down
 
         map.destroy();
-        });
     }
     
     function test_getGridData(t) {