Strip b2c/swrast test containers, lots of build-script cleanup
requested to merge daniels/mesa:you-know-when-you-get-a-box-thats-just-full-of-air-and-unnecessary-packing-material into main
Make the world use set -u when we're building, because it's pretty infuriating when you don't have it.
After that, strip down the test-gl and test-vk containers used at runtime, matching what we already do for LAVA to bin a bunch of stuff we don't need to unpack and instantiate 300 times every pipeline.