Most expensive item that can not be bought

#[test]
fn test_example1() {
    let start = std::time::Instant::now();
    let val = Solution::most_expensive_item(2, 5);
    let expected = 3;
    log_result!(val == expected, "test_example1", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn test_example2() {
    let start = std::time::Instant::now();
    let val = Solution::most_expensive_item(5, 7);
    let expected = 23;
    log_result!(val == expected, "test_example2", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn test_larger_primes() {
    let start = std::time::Instant::now();
    let val = Solution::most_expensive_item(3, 7);
    let expected = 11;
    log_result!(val == expected, "test_larger_primes", start, &expected, &val);
    assert_eq!(val, expected);
}

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

#[test]
fn test_twin_primes() {
    let start = std::time::Instant::now();
    let val = Solution::most_expensive_item(11, 13);
    let expected = 119;
    log_result!(val == expected, "test_twin_primes", start, &expected, &val);
    assert_eq!(val, expected);
}