Resolve duplicated_attributes clippy lint in test

    warning: duplicated attribute
      --> tests/common/mod.rs:1:10
       |
    1  | #![allow(dead_code)]
       |          ^^^^^^^^^
       |
    note: first defined here
      --> tests/test_precedence.rs:55:9
       |
    55 | #[allow(dead_code)]
       |         ^^^^^^^^^
    help: remove this attribute
      --> tests/common/mod.rs:1:10
       |
    1  | #![allow(dead_code)]
       |          ^^^^^^^^^
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#duplicated_attributes
       = note: `-W clippy::duplicated-attributes` implied by `-W clippy::all`
       = help: to override `-W clippy::all` add `#[allow(clippy::duplicated_attributes)]`

    warning: duplicated attribute
      --> tests/common/mod.rs:1:10
       |
    1  | #![allow(dead_code)]
       |          ^^^^^^^^^
       |
    note: first defined here
      --> tests/test_round_trip.rs:47:9
       |
    47 | #[allow(dead_code)]
       |         ^^^^^^^^^
    help: remove this attribute
      --> tests/common/mod.rs:1:10
       |
    1  | #![allow(dead_code)]
       |          ^^^^^^^^^
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#duplicated_attributes
       = note: `-W clippy::duplicated-attributes` implied by `-W clippy::all`
       = help: to override `-W clippy::all` add `#[allow(clippy::duplicated_attributes)]`

tests/test_precedence.rs

@@ -52,9 +52,7 @@ use std::sync::atomic::{AtomicUsize, Ordering};
 #[macro_use]
 mod macros;
 
-#[allow(dead_code)]
 mod common;
-
 mod repo;
 
 #[test]

tests/test_round_trip.rs

@@ -44,9 +44,7 @@ use std::time::Instant;
 #[macro_use]
 mod macros;
 
-#[allow(dead_code)]
 mod common;
-
 mod repo;
 
 #[test]