turn x86 in a test-only target

[japaric]

adds winfo! which does the same as info! but takes a Formatter argument
like write!

Now you can test the macros on x86. You cannot test info! directly (because of
global state) but winfo! does the same on Formatter

struct S {
    x: u8,
}

fn main() {
    let mut f = binfmt::Formatter::new();
    binfmt::winfo!(&mut f, "The answer is {:?}", S { x: 42 });
    println!("{:?}", f.bytes());
    // output:   [0, 0, 1, 42]
    // 'answer is'^  ^  ^  ^^ `S.x`
    //       timestamp  'S {{ x: {:u8} }}

    // note: no timestamp
    let mut f = binfmt::Formatter::new();
    binfmt::write!(&mut f, "The answer is {:u8}", 42);
    println!("{:?}", f.bytes());
    //    output: [2, 42]
    // 'answer is' ^  ^^ arg0
}