Deal with recent Chrome optimizations to avoid failing TileManager and tests

Chrome 27 seems to have added some optimizations that defer or even prevent loadend events for images that are not visible. This breaks some tests, and with the TileManager some maps with slow loading layers [don't work reliable](http://lists.osgeo.org/pipermail/openlayers-users/2013-June/028467.html) any more either. The TileManager problem can be fixed by simplifying the logic we use to accept tiles from the cache for being reused, and the tests can be fixed by pretending tiles are loaded.