Allow to run assert_de_tokens_error on token sequence that is not expected by enum deserializer
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-rs/serde