Fix reftests.

`dirname()` now returns a `&'a [u8]` instead of a `Path`, which was serialized
to `[ 1 2 3 4 5 ]`.
auto merge of #1114 : metajack/servo/reftest-sha-fix, r=larsbergstrom

`dirname()` now returns a `&'a [u8]` instead of a `Path`, which was serialized
to `[ 1 2 3 4 5 ]`.