fix clippy::derive_partial_eq_without_eq in tests

Author: hellow554

tests/test_core.rs

@@ -234,7 +234,7 @@ fn count_clones() {
     // Check that RepeatN only clones N - 1 times.
 
     use core::cell::Cell;
-    #[derive(PartialEq, Debug)]
+    #[derive(PartialEq, Eq, Debug)]
     struct Foo {
         n: Cell<usize>
     }