Added likely location of test images to git ignore file.

I'm hitting a scenario where running tests creates a directory called `iris_image_test_output` in the directory I run the tests from, which is mostly the iris source directory. I guess this shouldn't happen because the preferred location (and the one already ignored) is writeable, but for some reason `os.access()` doesn't think so (perhaps something weird with NFS setup). Anyway, this seemed like the most obvious shortest path to resolving this annoyance while I get on with other tasks!