test case use_alias_doesnt_exist: Add `#allow(unused_imports)`.

To suppress warning:

```
warning: unused import: `::std::num::NonZeroU8 as X`
   --> crubit/common/item_exists_test.rs:270:21
    |
270 |             pub use ::std::num::NonZeroU8 as X;
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(unused_imports)]` on by default

warning: 1 warning emitted
```
PiperOrigin-RevId: 579002929
Change-Id: Iabe28aa7a9d51aba9ce4e15cd6df17ecc83821fd
diff --git a/common/item_exists_test.rs b/common/item_exists_test.rs
index bcc33e5..b2cc114 100644
--- a/common/item_exists_test.rs
+++ b/common/item_exists_test.rs
@@ -266,6 +266,7 @@
     #[test]
     fn use_alias_doesnt_exist() {
         mod m {
+            #[allow(unused_imports)]
             pub use ::std::num::NonZeroU8 as X;
         }
         assert!(!item_exists::value_exists!(m::X));