Testing out multiple independent output states

Before actually writing some tests that do that we first need the following addressed:

• expand our ability to independently control outputs, should be taken care of in !1047 (merged)
• have the ability to specify multiple outputs, in VKMS, which we use in CI: https://lore.kernel.org/dri-devel/20221202142051.136651-1-marius.vlad@collabora.com/
• teach CI to make use of multiple outputs, and fix an issue I've found with it: !1066 (merged)

After all that gets in, we can then create plug-in tests, and address some of the scenarious detailed in !1047 (comment 1638354).