Substrings that begin and end with the same letter

#[test]
fn t1() {
    let start = std::time::Instant::now();
    let val = Solution::number_of_substrings("abcba".to_string());
    let expected = 7;
    log_result!(val == expected, "t1", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn t2() {
    let start = std::time::Instant::now();
    let val = Solution::number_of_substrings("abacad".to_string());
    let expected = 9;
    log_result!(val == expected, "t2", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn t3() {
    let start = std::time::Instant::now();
    let val = Solution::number_of_substrings("a".to_string());
    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::number_of_substrings("aaaa".to_string());
    let expected = 10;
    log_result!(val == expected, "t4", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn t5() {
    let start = std::time::Instant::now();
    let val = Solution::number_of_substrings("ab".to_string());
    let expected = 2;
    log_result!(val == expected, "t5", start, &expected, &val);
    assert_eq!(val, expected);
}