Ignore clippy::unit_arg lint in bench

```
warning: passing a unit value to a function
  --> crossbeam-skiplist/benches/skiplist.rs:97:13
   |
97 |             black_box(map.remove(&num, guard).unwrap().release(guard));
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unit_arg
   = note: `#[warn(clippy::unit_arg)]` on by default
help: move the expression in front of the call and replace it with the unit literal `()`
   |
97 ~             map.remove(&num, guard).unwrap().release(guard);
98 ~             black_box(());
   |
```

crossbeam-skiplist/benches/skiplist.rs

@@ -1,4 +1,5 @@
 #![feature(test)]
+#![allow(clippy::unit_arg)]
 
 extern crate test;