Fix Clippy errors in variant serialize_with tests

test_suite/tests/test_annotations.rs

@@ -850,7 +850,7 @@ fn deserialize_i8_as_unit_variant<'de, D>(deserializer: D) -> Result<(), D::Erro
     }
 }
 
-fn serialize_variant_as_string<S>(f1: &String,
+fn serialize_variant_as_string<S>(f1: &str,
                                   f2: &u8,
                                   serializer: S)
                                   -> Result<S::Ok, S::Error>

test_suite/tests/test_gen.rs

@@ -545,7 +545,7 @@ pub fn deserialize_some_unit_variant<'de, D>(_: D) -> StdResult<(), D::Error>
     unimplemented!()
 }
 
-pub fn serialize_some_other_variant<S>(_: &String, _: &u8, _: S) -> StdResult<S::Ok, S::Error>
+pub fn serialize_some_other_variant<S>(_: &str, _: &u8, _: S) -> StdResult<S::Ok, S::Error>
     where S: Serializer,
 {
     unimplemented!()