Use correct tokens in test to represent an enum variant

[Mingun]

The test that is fixing by this PR was introduces in #933 and has a purpose to reach the Content::Str case here that was fixed by that PR: https://github.com/serde-rs/serde/blob/05a5b7e3c6de502d45597cbc083f28bc1d4f4626/serde/src/private/de.rs#L1446

The Content::Str case here produced using visit_borrowing_str here (that the only case when Content::Str can be created): https://github.com/serde-rs/serde/blob/05a5b7e3c6de502d45597cbc083f28bc1d4f4626/serde/src/private/de.rs#L424-L429

which is called as a response to Token::BorrowedStr: https://github.com/serde-rs/test/blob/d1294b3ad549874d5035752ac62b4eb75cee5060/src/de.rs#L129

However, unit variants of externally tagged enum cannot be deserialized from the string token by itself. The one of following combinations of tokens should be used to correctly represent enum variant:

• [xxxVariant { .. }]
• [Enum { .. }, xxxVariant { variant, .. }]
• [Enum { .. }, String(variant), <variant content>]
• [Enum { .. }, Str(variant), <variant content>]
• [Enum { .. }, BorrowedStr(variant), <variant content>]

Because we want to reach Content::Str case, the only possible representation which we can use is [Enum { name: "Level" }, BorrowedStr("Info"), Unit] (other variants will be captured as Content::String).