ci: Enable -Werror on meson-vulkan and meson-testing.

ci: Enable -Werror on meson-vulkan and meson-testing.

I want to make sure that I don't introduce warnings in turnip where we
have active work going on, and I also want to make sure that the drivers
we care about testing are warnings-clean.

As with the previous -Werror change, this is for CI only and doesn't
affect end-user builds.

Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3607>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3607>