tests: Use public-code and private-code instead of code

The wayland-scanner sub-commands private-code and public-code replaced
the old code command, so lets use those in the tests instead.

This requires at least wayland-scanner 1.15.0.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Simon Ser <contact@emersion.fr>