move extra::test to libtest

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.

### Not ready to be merged. Need help.

Now, it seems most source code compiles OK, and most tests passed, except the tests of `libtest` itself got compile error:

```
oxidize: x86_64-unknown-linux-gnu/stage2/test/testtest-x86_64-unknown-linux-gnu
/home/liigo/rust/rust/src/libtest/lib.rs:1:1: 1:1 error: unresolved import: there is no `test` in `???`
/home/liigo/rust/rust/src/libtest/lib.rs:1 // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
                                           ^
/home/liigo/rust/rust/src/libtest/lib.rs:1:1: 1:1 error: failed to resolve import `test`
/home/liigo/rust/rust/src/libtest/lib.rs:1 // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
                                           ^
error: aborting due to 2 previous errors
make: *** [x86_64-unknown-linux-gnu/stage2/test/testtest-x86_64-unknown-linux-gnu] Error 101
```

Sadly, I don't know to fix it. Can anyone help me? Thanks!