tests/pixbuf-randomly-modified: Drop iterations to 10
tests/pixbuf-randomly-modified: Drop iterations to 10

1000 iterations takes far too long for automated testing;
we'll get incremental coverage over time even with just
10, and the test can of course be run manually.

GNOME/gdk-pixbuf,Distrotech/gdk-pixbuf,Distrotech/gdk-pixbuf,YueLinHo/MB3Gdk-pixbuf,GNOME/gdk-pixbuf,YueLinHo/MB3Gdk-pixbuf,GNOME/gdk-pixbuf,YueLinHo/MB3Gdk-pixbuf,YueLinHo/MB3Gdk-pixbuf,Distrotech/gdk-pixbuf,Distrotech/gdk-pixbuf,Distrotech/gdk-pixbuf,GNOME/gdk-pixbuf,YueLinHo/MB3Gdk-pixbuf