Move all untagged enum tests (except flatten) into a dedicated module

Moved and renamed:
From test_annotations
- test_expecting_message_untagged_tagged_enum                  => expecting_message
- flatten::enum_::untagged::straitforward                      => contains_flatten

From test_macros
- test_untagged_newtype_struct                                 => newtype_struct
- test_untagged_enum                                           => complex
- test_untagged_enum_with_flattened_integer_key                => contains_flatten_with_integer_key
- test_enum_in_untagged_enum                                   => newtype_enum
- test_untagged_bytes                                          => string_and_bytes
- test_untagged_newtype_variant_containing_unit_struct_not_map => newtype_unit_and_empty_map

test_suite/tests/test_annotations.rs

@@ -1895,18 +1895,6 @@ fn test_expecting_message_externally_tagged_enum() {
     );
 }
 
-#[test]
-fn test_expecting_message_untagged_tagged_enum() {
-    #[derive(Deserialize)]
-    #[serde(untagged)]
-    #[serde(expecting = "something strange...")]
-    enum Enum {
-        Untagged,
-    }
-
-    assert_de_tokens_error::<Enum>(&[Token::Str("Untagged")], "something strange...");
-}
-
 #[test]
 fn test_expecting_message_identifier_enum() {
     #[derive(Deserialize)]
@@ -2958,41 +2946,6 @@ mod flatten {
         mod untagged {
             use super::*;
 
-            #[test]
-            fn straightforward() {
-                #[derive(Serialize, Deserialize, PartialEq, Debug)]
-                #[serde(untagged)]
-                enum Data {
-                    A {
-                        a: i32,
-                        #[serde(flatten)]
-                        flat: Flat,
-                    },
-                }
-
-                #[derive(Serialize, Deserialize, PartialEq, Debug)]
-                struct Flat {
-                    b: i32,
-                }
-
-                let data = Data::A {
-                    a: 0,
-                    flat: Flat { b: 0 },
-                };
-
-                assert_tokens(
-                    &data,
-                    &[
-                        Token::Map { len: None },
-                        Token::Str("a"),
-                        Token::I32(0),
-                        Token::Str("b"),
-                        Token::I32(0),
-                        Token::MapEnd,
-                    ],
-                );
-            }
-
             #[derive(Debug, PartialEq, Serialize, Deserialize)]
             struct Flatten {
                 #[serde(flatten)]