Add few unit tests for Caca::Display
user
```ruby
require 'test/unit'
require 'caca'

class TC_Canvas < Test::Unit::TestCase
    def test_create
	d = Caca::Display.new
        assert_not_nil(d, 'Display creation failed')
    end
    def test_create_with_driver
	d = Caca::Display.new(nil, Caca::Display.driver_list[0])
        assert_not_nil(d, 'Display creation failed')
    end
    def test_create_from_canvas
        c = Cucul::Canvas.new(3, 3)
	d = Caca::Display.new(c)
        assert_not_nil(d, 'Display creation failed')
	assert_equal(d.canvas, c, 'Wrong canvas')
    end
    def test_set_title
        c = Cucul::Canvas.new(3, 3)
        d = Caca::Display.new(c)
        d.title = "Test !"
    end
end
```
assistant