I don't think `extra` is a good/meaningful name for a library. `libextra` should disappear, and we move all of its sub modules out of it. This PR is just one of that steps: move `extra::test` to `libtest`.
I didn't add `libtest` to doc index, because it's an internal library currently.
**Update:**
All comments addressed. All tests passed. Rebased and squashed.