Count submatrices with equal frequency of x and y

#[test]
fn test_all_dots() {
    let start = std::time::Instant::now();
    let grid = g(&["...", "...", "..."]);
    let val = Solution::number_of_submatrices(grid);
    let expected = 0;
    log_result!(val == expected, "test_all_dots", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn test_example1() {
    let start = std::time::Instant::now();
    let grid = g(&["XY.", "Y.."]);
    let val = Solution::number_of_submatrices(grid);
    let expected = 3;
    log_result!(val == expected, "test_example1", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn test_example2() {
    let start = std::time::Instant::now();
    let grid = g(&["XX", "XY"]);
    let val = Solution::number_of_submatrices(grid);
    let expected = 0;
    log_result!(val == expected, "test_example2", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn test_example3() {
    let start = std::time::Instant::now();
    let grid = g(&["..", ".."]);
    let val = Solution::number_of_submatrices(grid);
    let expected = 0;
    log_result!(val == expected, "test_example3", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn test_single_x() {
    let start = std::time::Instant::now();
    // "X" alone: 1 X, 0 Y -> not equal
    let grid = g(&["X"]);
    let val = Solution::number_of_submatrices(grid);
    let expected = 0;
    log_result!(val == expected, "test_single_x", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn test_xy_top_left() {
    let start = std::time::Instant::now();
    // [[X,Y]] -> submatrix [0][0..1]: 1X 1Y -> count=1; [0][0]: 1X 0Y -> 0
    let grid = g(&["XY"]);
    let val = Solution::number_of_submatrices(grid);
    let expected = 1;
    log_result!(val == expected, "test_xy_top_left", start, &expected, &val);
    assert_eq!(val, expected);
}