flatpak: Adapt to changes in mutter Meson options

Adapt to changes introduced in
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3812.

Signed-off-by: Adrien Plazas <aplazas@gnome.org>

build-aux/flatpak/org.gnome.Settings.Devel.json

@@ -709,8 +709,7 @@
                 "-Dsm=false",
                 "-Dintrospection=true",
                 "-Ddocs=false",
-                "-Dtests=false",
-                "-Dnative_tests=false",
+                "-Dtests=disabled",
                 "-Dprofiler=false"
             ],
             "sources" : [