BSD cross test support

[Dylan-DPC-zz]

• x86_64-unknown-dragonfly
• x86_64-unknown-freebsd
• x86_64-unknown-netbsd

and, the support matrix, shows that none of these can run tests.

Originally posted by @gnzlbg in #40 (comment)

[gnzlbg]

@malbarbo IIRC the libc crate does run tests in freebsd under qemu-system, is that correct ?

[malbarbo]

Yes, that's correct.

To run the tests a disk image with the test files are created, then the qemu is booted with the os image and the disk test image, finally a custom init scrip run the tests. See https://github.com/rust-lang/libc/blob/master/ci/run.sh#L43

The qemu image used by libc was manually created. I think we would want an automatically build one.

[asomers]

Running tests on FreeBSD under QEMU is slow and awkward. Using www.cirrus-ci.com (available since about January 2019) is a much better option.

[Alexhuszagh]

Closing this, since it's difficult without full system emulation, which is quite slow. We can't ship FreeBSD images with Linux binary compatibility either, due to issues with running FreeBSD images inside Docker. Therefore, it's up to the user to get their own FreeBSD instance up and running, such as on a cloud service.

We'll add to the wiki how to run the tests, but I think shipping a full FreeBSD image is beyond the scope of cross.