N th tribonacci number

#[test]
fn t1() {
    let start = std::time::Instant::now();
    let val = Solution::tribonacci(4);
    let expected = 4;
    log_result!(val == expected, "t1", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn t2() {
    let start = std::time::Instant::now();
    let val = Solution::tribonacci(25);
    let expected = 1389537;
    log_result!(val == expected, "t2", start, &expected, &val);
    assert_eq!(val, expected);
}

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

#[test]
fn t4() {
    let start = std::time::Instant::now();
    let val = Solution::tribonacci(1);
    let expected = 1;
    log_result!(val == expected, "t4", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn t5() {
    let start = std::time::Instant::now();
    let val = Solution::tribonacci(2);
    let expected = 1;
    log_result!(val == expected, "t5", start, &expected, &val);
    assert_eq!(val, expected);
}