Use a custom Rust test framework

[jdreaver]

I'm trying to follow along with https://os.phil-opp.com/testing/, but I'm using limine instead of the bootloader crate, so I have to roll my own a bit.

Pending TODOs:

• Somehow link the test binaries to dedicated ISO files to run in QEMU
  • Use cargo test --no-run --message-format=json along with jq to fine the produced binaries
  • Move building the ISO with limine into a script. Make sure to tell limine to wait 0 seconds to boot.
• Add QEMU exit after tests run.
• Step back and ensure I actually want to do it this way 😄 I tend to think this is more trouble than its worth. Pure code we can unit test should go into library crates that are unit tested on the host machine, and integration tests will almost certainly need dedicated setup and execution code that isn't helped much by Rust test frameworks.