Add a test OS

[toku-sa-n]

The code is taken from https://github.com/toku-sa-n/ramen/tree/00193f21cdb72f37e5b4059a1e47c9be88992f5c. It is GPL-licensed, but this revision does not include contributions from the other contributors except bots and me, and thus it does not violate GPL as I'm the author of it.

• Add a changelog?
• Set up CI
• Manually build QEMU with -DDEBUG_XHCI on CI with debug mode enabled
• Add comments to refer xHCI specification for each step?
• Is this actually necessary?

[toku-sa-n]

I'm in doubt if we really need a test OS.

• It has lots of code and is too difficult to read through.
• We can test just a few structures. All the others are left untested.
• First and foremost, what do we test? Structures have correct field getters/setters?