Count digit appearances

#[test]
fn example1() {
    let start = std::time::Instant::now();
    let val = Solution::count_digit_occurrences(vec![12, 54, 32, 22], 2);
    let expected = 4;
    log_result!(val == expected, "example1", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn example2() {
    let start = std::time::Instant::now();
    let val = Solution::count_digit_occurrences(vec![1, 34, 7], 9);
    let expected = 0;
    log_result!(val == expected, "example2", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn large_number() {
    let start = std::time::Instant::now();
    let val = Solution::count_digit_occurrences(vec![999999], 9);
    let expected = 6;
    log_result!(val == expected, "large_number", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn mixed_digits() {
    let start = std::time::Instant::now();
    let val = Solution::count_digit_occurrences(vec![123, 456, 789], 5);
    let expected = 1;
    log_result!(val == expected, "mixed_digits", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn repeated_digit() {
    let start = std::time::Instant::now();
    let val = Solution::count_digit_occurrences(vec![7777], 7);
    let expected = 4;
    log_result!(val == expected, "repeated_digit", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn single_digit_match() {
    let start = std::time::Instant::now();
    let val = Solution::count_digit_occurrences(vec![5], 5);
    let expected = 1;
    log_result!(val == expected, "single_digit_match", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn zero_digit() {
    let start = std::time::Instant::now();
    let val = Solution::count_digit_occurrences(vec![100, 1000], 0);
    let expected = 5;
    log_result!(val == expected, "zero_digit", start, &expected, &val);
    assert_eq!(val, expected);
}