Checkbox should be running tests with both Xorg and Wayland

### Enhancement Proposal

Running Wayland vs Xorg exposes different code paths to a lot of the software stack. Some issues only come up in Wayland, others only in Xorg and GDM allows switching between the two stacks.

To help expose issues that are unique to one or the other, at least some subset of tests should be run on both.

Example issues:

1. Nouveau fails with IOMMU enabled, but [only in Wayland](https://gitlab.freedesktop.org/drm/nouveau/-/issues/225).
2. amdgpu fails to resume from s2idle with [Wayland due to an IOCTL](https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2020685) that mutter uses only in Wayland.
3. CPU core frequency is locked to 512MHz due to a [VCN initialization issue](https://github.com/torvalds/linux/commit/167be8522821f), specific to Xorg.

Start by tracing how Checkbox runs its tests and how the testing environment selects between Wayland and Xorg. Define which subset should run in both sessions and how success will be verified; the issue names no files or tests, so the relevant entry points must first be identified.

Written by the indexing model from the issue text.