Work around dead_code warning in flock implementation

    warning: field `0` is never read
      --> src/flock.rs:22:12
       |
    22 |     Locked(MutexGuard<'static, ()>),
       |     ------ ^^^^^^^^^^^^^^^^^^^^^^^
       |     |
       |     field in this variant
       |
       = note: `#[warn(dead_code)]` on by default
    help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
       |
    22 |     Locked(()),
       |            ~~

src/flock.rs

@@ -19,7 +19,7 @@ pub struct Lock {
 // integration test crate.
 enum Guard {
     NotLocked,
-    Locked(MutexGuard<'static, ()>),
+    Locked(#[allow(dead_code)] MutexGuard<'static, ()>),
 }
 
 // Best-effort filesystem lock to coordinate different #[test] functions across