Update sudoku.rb with all tests

Update sudoku.rb with all tests

* Test that is invalid if all elements equal to zero
* Test that is invalid if any element is repeated in the same column
* Test that is invalid if any element is repeated in the same row
* Test that is valid if no element is repeated in the same column and row
Os testes podem ser refatorados para a seguinte forma:

```ruby
class TestSudoku < Minitest::Test
  describe "Array" do
    it "is invalid when is empty" do
      arr = Array.new(9) { Array.new(9) { 0 } }

      assert_equal false, Sudoku.problem(arr)
    end

    describe "when have same number" do
      before do
        @arr = Array.new(9) { Array.new(9) { |i| i+1 } }
      end

      it "is interating horizontally" do
        assert_equal false, Sudoku.problem(@arr)
      end

      it "is interating vertically" do
        assert_equal false, Sudoku.problem(@arr)
      end
    end

    it "is valid" do
      arr = [[1,2,3,4,5,6,7,8,9],
             [2,3,4,5,6,7,8,9,1],
             [3,4,5,6,7,8,9,1,2],
             [4,5,6,7,8,9,1,2,3],
             [5,6,7,8,9,1,2,3,4],
             [6,7,8,9,1,2,3,4,5],
             [7,8,9,1,2,3,4,5,6],
             [8,9,1,2,3,4,5,6,7],
             [9,1,2,3,4,5,6,7,8]]

      assert_equal true, Sudoku.problem(arr)
    end
  end
end
```