Add some test support for BSDs

[KodrAus]

Part of #78

This PR attempts to use https://github.com/vmactions/freebsd-vm to run tests in CI on a FreeBSD target. It appears to use VirtualBox virtualization on a MacOS host.

If I manage to get things working on this target then I'll try NetBSD and Solaris as well.

[KodrAus]

Looks like we've got some tests running in FreeBSD along with a failure: https://github.com/rust-lang/socket2/runs/1679158951?check_suite_focus=true#step:3:1567

failures:

---- only_v6 stdout ----
thread 'main' panicked at 'assertion failed: `(left != right)`
  left: `true`,
 right: `true`: initial value and argument are the same', tests/socket.rs:772:1
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[Thomasdezeeuw]

@KodrAus fixed the test issue in #196.

[Thomasdezeeuw]

Two small things, but LGTM otherwise. Can you rebase this on master to check if I fixed the ipv6_only test failure with #196.

[KodrAus]

Done! Let's see if it passes now.

You may also be interested in vmactions/freebsd-vm#8, which explains why the FreeBSD output contains a lot of garbled text right now before it gets to our code.

[KodrAus]

Looks like we're green now.

If you like I should be able to follow this one up with NetBSD and Solaris too.

[Thomasdezeeuw]

@KodrAus thanks. Adding NetBSD and Solaris would be great.