* Removes an unneeded `#![warn(clippy::extra_unused_lifetimes)]` from
the `needless_lifetimes` test - the tests for those are in a different
file.
* Renames `unused_lt.{rs,stderr}` to `extra_unused_lifetimes.{rs,stderr}`
* Renames `lifetimes.{rs,stderr}` to `needless_lifetimes.{rs,stderr}`
