Run 'cargo test' for the Rust gdk-pixbuf-loader
Run 'cargo test' for the Rust gdk-pixbuf-loader

The binary is not installed yet via autotools, but let's run a build/test
of the new gdk-pixbuf-loader in the CI to ensure it keeps working.

Part-of: <https://gitlab.gnome.org/GNOME/librsvg/-/merge_requests/749>

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg