Find the difference

#[test]
fn test_added_at_end() {
    let start = std::time::Instant::now();
    let val = Solution::find_the_difference("xyz".to_string(), "xyzw".to_string());
    let expected = 'w';
    log_result!(val == expected, "test_added_at_end", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn test_duplicate_character() {
    let start = std::time::Instant::now();
    let val = Solution::find_the_difference("a".to_string(), "aa".to_string());
    let expected = 'a';
    log_result!(val == expected, "test_duplicate_character", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn test_example_1() {
    let start = std::time::Instant::now();
    let val = Solution::find_the_difference("abcd".to_string(), "abcde".to_string());
    let expected = 'e';
    log_result!(val == expected, "test_example_1", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn test_example_2() {
    let start = std::time::Instant::now();
    let val = Solution::find_the_difference("".to_string(), "y".to_string());
    let expected = 'y';
    log_result!(val == expected, "test_example_2", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn test_multiple_same_characters() {
    let start = std::time::Instant::now();
    let val = Solution::find_the_difference("ae".to_string(), "aea".to_string());
    let expected = 'a';
    log_result!(val == expected, "test_multiple_same_characters", start, &expected, &val);
    assert_eq!(val, expected);
}