Add `hr` test
Normalize `hr`

Firefox uses different `box-sizing` and `height` values to all other
browsers. Firefox doesn't currently support `box-sizing` without the
`-moz-` prefix, so we use both the vendor-prefixed and unprefixed
properties to ensure that it matches the `content-box` value of other
browsers. It also requires the `height` to be set to `0`.

Known issue: this does not normalize IE 6/7 as `hr` cannot be reliably
styled in those browsers.