test cases, with fully up-to-date artful + https://launchpad.net/~osomon/+archive/ubuntu/test-fix-1718446/+packages:

 - log into ubuntu session, disable wayland, reboot, verify that it falls back to ubuntu, re-enable wayland, reboot, verify that ubuntu session is selected by default and used when logging in
 - log out, select ubuntu-xorg, log in, disable wayland, reboot, see that no session is selected by default (bug), but verify that logging in uses ubuntu-xorg (DESKTOP_SESSION=ubuntu-xorg), re-enable wayland, reboot, verify that ubuntu-xorg is selected and logged into
 - verify that when wayland is enabled, the following sessions are listed: GNOME, GNOME on Xorg, Ubuntu, Ubuntu on Xorg, Unity
 - verify that when wayland is disabled, only the following sessions are listed: GNOME on Xorg, Ubuntu on Xorg, Unity
 - disable wayland, select "Ubuntu on Xorg" and log in, verify that DESKTOP_SESSION=ubuntu, re-enable wayland and reboot, verify that "Ubuntu" is selected by default
 - with wayland enabled, log into Unity, disable wayland and reboot, verify that Unity is selected and logged into
 - log out, select "GNOME on Xorg", log in, verify that DESKTOP_SESSION=gnome, re-enable wayland, reboot, verify that "GNOME" is selected by default, select "GNOME on Xorg", log in, verify that DESKTOP_SESSION=gnome-xorg, disable wayland, reboot, verify that no session is selected by default (bug), but that logging in without selecting any session logs into gnome-xorg, log out, explicitly select "GNOME on Xorg", log in, verify that DESKTOP_SESSION=gnome, re-enable wayland, reboot, verify that "GNOME" is selected by default