It looks like the tests currently don't do anything. I added a panic inside the #[cfg(feature = "bios")] block inside run_test_kernel_internal and it still passes even with the bios feature explicitly enabled. I think this is because features do not get passed on to the test runner crate.
I fixed this by adding a feature dependency, but I'm not entirely sure if that is the proper way to do it. The tests take considerably longer now, so I think that's good.

[features]
default = ["bios", "uefi"]
bios = ["dep:mbrman","bootloader_test_runner/bios"]
uefi = ["dep:gpt","bootloader_test_runner/uefi"]