Allow unused code in tests

To avoid working around some code being unused in tests due to it being stubbed out with `#[cfg(test)]`.

src/bootstrap/src/lib.rs

@@ -15,6 +15,7 @@
 //!
 //! More documentation can be found in each respective module below, and you can
 //! also check out the `src/bootstrap/README.md` file for more information.
+#![cfg_attr(test, allow(unused))]
 
 use std::cell::{Cell, RefCell};
 use std::collections::{BTreeSet, HashMap, HashSet};