Mechanism for running root task tests with less Nix

[nspin]

The idea would be to have a build system (probably still Nix) generate a Cargo config.toml and test runner script for a matrix of configurations. The config.toml scripts would set build.target and target.*.runner, along with env.SEL4_PREFIX (etc.), and the test runner would bundle the test root task into the kernel loader, and then run the bundle via QEMU.

Then running a test for a particular configuration would be as simple as passing a cargo test --config <config.toml> -p foo-crate.