Due to a limitation, bug, feature or whatever you call this in pacman, the upgrade path for testing users to the new xorg will be a bit troublesome. Pacman will complain about existing files in /usr/share/X11/xkb/compiled for xorg-server and existing files in /etc/X11/xdm for the xorg-xdm package.
A complicated workaround:
pacman -Sy pacman -Rd xorg-server xorg-clients pacman -Sf xorg-xdm pacman -S xorg-clients xorg-server pacman -Su
If you use xdm and you have made customizations to /etc/X11/xdm configuration files, make sure you make backups of them before performing these operations.
Another bug might show up in pacman after the upgrade of xorg-clients: several files which used to be in xorg-clients are gone due to movements between packages. To resolve this, reinstall all packages xorg-clients depends on.