ArtworkManagerTests: make test more robust

It could happen that this test failed in case banshee had not
ever been run in the computer, which was assuming the cache
folder to exist, and would fail this way:

GLib.GException : Failed to open
'/home/knocte/.cache/media-art/36/album-d33f25dbd7dfb4817a7e99f6bc2de49e.jpg'
for writing: No such file or directory
 at Gdk.Pixbuf.Save (System.String filename, System.String type)
 at Banshee.Collection.Gui.Tests.ArtworkManagerTests.TestsSizePath ()