Add more tests for flatten enums to increase test coverage

[Mingun]

This PR adds more tests for enum flatten into the struct (each in their own commit):

• Ensure, that serialization produces output that can be deserialized
• Ensure, that any order of fields in the serialized form for

  struct Flatten {
    #[serde(flatten)]
    data: Enum,
    other_fields: ...,
  }

  can be deserialized
• Ensure, that Enum::Unit can be deserialized when Enum is flatten like above