Count pairs whose sum is less than target

#[test]
fn t1() {
    let start = std::time::Instant::now();
    let val = Solution::count_pairs(vec![-1, 1, 2, 3, 1], 2);
    let expected = 3;
    log_result!(val == expected, "t1", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn t2() {
    let start = std::time::Instant::now();
    let val = Solution::count_pairs(vec![-6, 2, 5, -2, -7, -1, 3], -2);
    let expected = 10;
    log_result!(val == expected, "t2", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn t3() {
    let start = std::time::Instant::now();
    let val = Solution::count_pairs(vec![1, 2], 4);
    let expected = 1;
    log_result!(val == expected, "t3", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn t4() {
    let start = std::time::Instant::now();
    let val = Solution::count_pairs(vec![1, 1, 1], 3);
    let expected = 3;
    log_result!(val == expected, "t4", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn t5() {
    let start = std::time::Instant::now();
    let val = Solution::count_pairs(vec![5, 5], 10);
    let expected = 0;
    log_result!(val == expected, "t5", start, &expected, &val);
    assert_eq!(val, expected);
}