rustdoc: Use --crate-name with --test

This ensures that the name of the crate is set from the command line for tests
so the auto-injection of `extern crate <name>` in doc tests works correctly.
Or `match crate_name { Some(n) => krate.name = n, None => {} }` or `for n in crate_name.move_iter() { krate.name = n }` or `crate_name.map(|n| krate.name = n)`.