Increase the "pixbuf-randomly-modified" test timeout
I am trying to build the gdk-pixbuf on RISC-V board. It is successfully compiled, but it failed with the "pixbuf-randomly-modified" test cause the performance of the RISC-V board is not well. And after some retry, I found the test needs 400-600s to finish. So I think 1000 might be enough for the test.
The file to update: https://gitlab.gnome.org/GNOME/gdk-pixbuf/-/blob/master/tests/meson.build#L206