tests: corpus of test kernels

[fabianfreyer]

We need a corpus of test kernels to test with. There's a few options we could go with here:

Non-functional kernels:

• ELF: Build a simple ELF stub and add a multiboot / multiboot2 header programmatically
• a.out kludge: Use random data with a multiboot / multiboot2 header containing address data

Functional test kernels:

• ELF without a.out kludge: test kernels in grub2. These can be found in grub-core/tests/boot/kernel-${ARCH}.S, and can be built using make multiboot.elf and make multiboot2.elf, respectively. These are licensed under the GPL though, not sure if they can simply be incorporated.
• ELF with a.out kludge: Test kernels built with rumprun.
• test kernel without a.out kludge: write one

[fabianfreyer]

the qemu test kernel is MIT-Licensed.