Add link to libtest in doc index

Author: lilyball

src/doc/index.md

@@ -43,6 +43,7 @@ li {list-style-type: none; }
 * [The `sync` library for concurrency-enabled mechanisms and primitives](sync/index.html)
 * [The `syntax` library, the Rust parser](syntax/index.html)
 * [The `term` terminal-handling library](term/index.html)
+* [The `test` library](test/index.html)
 * [The `uuid` 128-bit universally unique identifier library](uuid/index.html)
 
 # Tooling