t/GdkPixbufLoader.t: Use a bigger file to test chunking

gdk-pixbuf 2.30.3 unified sniff buffer sizes across loaders, effectively
meaning that we now don't get any signals until at least 4096 bytes have
been written to the loader.  Use a bigger test file to work around this.

https://bugzilla.gnome.org/show_bug.cgi?id=725055