Allow to run assert_de_tokens_error on token sequence that is not expected by enum deserializer

Before that fix following code panics, because `Token::Unit` was unexpected by test deserializer:
```
#[derive(Deserialize)]
enum E { ... }

assert_de_tokens_error::<E>(&[Token::Unit], "...");
```

serde_test/src/de.rs

@@ -250,9 +250,7 @@ impl<'de, 'a> de::Deserializer<'de> for &'a mut Deserializer<'de> {
             {
                 visitor.visit_enum(DeserializerEnumVisitor { de: self })
             }
-            _ => {
+            _ => self.deserialize_any(visitor)
-                unexpected!(self.next_token());
-            }
         }
     }