changed image_tester so it doesn't try to overwrite frozen classes
changed image_tester so it doesn't try to overwrite frozen classes

Eric89GXL/vispy,michaelaye/vispy,ghisvail/vispy,Eric89GXL/vispy,michaelaye/vispy,ghisvail/vispy,drufat/vispy,ghisvail/vispy,drufat/vispy,michaelaye/vispy,drufat/vispy,Eric89GXL/vispy