Subprocess test

Introduces macro and some infrastructure that allows running your test code in separate subprocess and verify it, if needed, inside test invocation. See crate documentation for details.

Some small examples:

subprocess_test::subprocess_test! {
    #[test]
    fn just_success() {
        let value = 1;
        assert_eq!(value + 1, 2);
    }
    /// Custom validation of output
    #[test]
    fn one_plus_one() {
        println!("{}", 1 + 1);
    }
    verify |success, output| {
        assert!(success);
        assert_eq!(output, "2\n");
    }
}