Count substrings starting and ending with given character

#[test]
fn t1() {
    let start = std::time::Instant::now();
    let val = Solution::count_substrings("abada".to_string(), 'a');
    let expected = 6;
    log_result!(val == expected, "t1", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn t2() {
    let start = std::time::Instant::now();
    let val = Solution::count_substrings("zzz".to_string(), 'z');
    let expected = 6;
    log_result!(val == expected, "t2", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn t3() {
    let start = std::time::Instant::now();
    let val = Solution::count_substrings("a".to_string(), 'a');
    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::count_substrings("abc".to_string(), 'b');
    let expected = 1;
    log_result!(val == expected, "t4", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn t5() {
    let start = std::time::Instant::now();
    let val = Solution::count_substrings("aaa".to_string(), 'a');
    let expected = 6;
    log_result!(val == expected, "t5", start, &expected, &val);
    assert_eq!(val, expected);
}