Added some html specific tests

1. Ox, currently, does not detect the doctype right. (Maybe a no-want feature)
2. Does not fire the text callback when not terminated text is present
3. Does not handle the not closed self-closing tags (hr, br) right. These tags must be closed immediately and not wait until a closing tag is found. E.g. The following html

```html
<html>A terminated <br> text</html>
```
should trigger the following callbacks:

```ruby
[
        [:start_element, :html],
        [:text, "A terminated "],
        [:start_element, :br],
        [:error, "Start End Mismatch: element 'br' not closed", 1, 23],
        [:end_element, :br],
        [:text, " text"],
        [:end_element, :html]
]
```
For all those tests and for the maintainability of the tests in general some refactoring took place. Added a `test/sax` directory where I have written the new `smart_test.rb` and copied the old `sax_test.rb` into a new `base_test.rb`.