Tests for grouping content based on corresponding volume

Addresses requirements:
- [ ] [4.6:90](http://snaekobbi.github.io/requirements#4.6:90)
- [ ] [4.6:91](http://snaekobbi.github.io/requirements#4.6:91)
- [ ] [4.6:92](http://snaekobbi.github.io/requirements#4.6:92)

Related issues:
- [ ] https://github.com/snaekobbi/braille-css-spec/issues/24