Add tests for span level renderer.

It seems span level API not working.

http://misaka.61924.nl/manual/

```
autolink(str link, bool is_email)
codespan(str code)
double_emphasis(str text)
emphasis(str text)
image(str link, str title, str alt_text)
linebreak()
link(str link, str title, str content)
raw_html(str raw_html)
triple_emphasis(str text)
strikethrough(str text)
superscript(str text)
```