test-gxps: do not use deprecated api
test-gxps: do not use deprecated api

See that there is no need to bump the gtk3 version since
the api that we are using is >= 3.0 which is the version
supported by the library.

https://bugzilla.gnome.org/show_bug.cgi?id=771057

GNOME/libgxps,GNOME/libgxps,GNOME/libgxps