Count substrings with only one distinct letter

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

#[test]
fn test_example1() {
    let start = std::time::Instant::now();
    let val = Solution::count_letters("aaaba".to_string());
    let expected = 8;
    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::count_letters("aaaaaaaaaa".to_string());
    let expected = 55;
    log_result!(val == expected, "test_example2", start, &expected, &val);
    assert_eq!(val, expected);
}

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

#[test]
fn test_two_runs() {
    let start = std::time::Instant::now();
    let val = Solution::count_letters("aabb".to_string());
    let expected = 6;
    log_result!(val == expected, "test_two_runs", start, &expected, &val);
    assert_eq!(val, expected);
}