WIP: Add autopkgtests for the SDK
These verify that we can build a simple SDL2 project with make, cmake and meson, and that we can run a simple SDL2 project with gdb (but for now only if we are not in Docker, because unprivileged Docker containers can't usually use ptrace).
TODO: Running these against the Docker image with autopkgtest will require https://salsa.debian.org/ci-team/autopkgtest/merge_requests/3 which isn't finished yet.
TODO: We should run these against the schroot.