test_socket: add AddressFamily::Vsock test
test_socket: add AddressFamily::Vsock test

The current VSOCK implementation does not support loopback devices,
so, for now, we expect a failure in the spawned thread when it
tries to connect.

Signed-off-by: Stefano Garzarella <6f3dd2004310421c0dda1107f4857e62a774d5df@redhat.com>

asomers/nix,nix-rust/nix,asomers/nix,geofft/nix-rust,geofft/nix-rust,carllerche/nix-rust,nix-rust/nix,carllerche/nix-rust,carllerche/nix-rust,geofft/nix-rust