Add test of const-generic unit struct where-clause edge case

Closes #2501.

test_suite/tests/test_gen.rs

@@ -760,6 +760,10 @@ fn test_gen() {
         #[serde(serialize_with = "vec_first_element")]
         vec: Vec<Self>,
     }
+
+    #[derive(Deserialize)]
+    #[serde(bound(deserialize = "[&'de str; N]: Copy"))]
+    struct GenericUnitStruct<const N: usize>;
 }
 
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