implement testing

[tsatke]

rust-osdev/bootloader#366

Seems to be harder than anticipated.

Goal is to have the kernel tests run in QEMU with a simple call to cargo test, no fuzz.

[tsatke]

Attempted to solve this with a proc macro and the https://github.com/dtolnay/inventory crate, but no luck, since that depends on the https://github.com/mmastrac/rust-ctor crate, and that doesn't support our target x86_64-unknown-none, or any custom target without a well-known target_os for that matter. (See https://github.com/mmastrac/rust-ctor/blob/fdaf51e7fd7d2bb010da088f67524a1236bdaabb/ctor/src/lib.rs#L173-L174)

[tsatke]

Resorted to small test kernels in 4e74843 , which allows tests to be run locally, but for some reason it fails on the GitHub actions runner.
However, since we have tests now, this issue can be closed.