Subtract the product and sum of digits of an integer

#[test]
fn t1() {
    let start = std::time::Instant::now();
    let val = Solution::subtract_product_and_sum(234);
    let expected = 15;
    log_result!(val == expected, "t1", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn t2() {
    let start = std::time::Instant::now();
    let val = Solution::subtract_product_and_sum(4421);
    let expected = 21;
    log_result!(val == expected, "t2", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn t3() {
    let start = std::time::Instant::now();
    let val = Solution::subtract_product_and_sum(1);
    let expected = 0;
    log_result!(val == expected, "t3", start, &expected, &val);
    assert_eq!(val, expected);
}