Can"t run `cargo test --all` for `error-chain` anymore. The tests don"t
compile because of `#[deny(invalid_doc_attributes)]`. Here"s  the error
message:

```
error: this attribute can only be applied at the crate level
   --> tests/tests.rs:508:7
    |
508 | #[doc(test)]
    |       ^^^^
    |
    = note: read <https://doc.rust-lang.org/nightly/rustdoc/the-doc-attribute.html#at-the-crate-level> for more information
    = note: `#[deny(invalid_doc_attributes)]` on by default
help: to apply to the crate, use an inner attribute
    |
508 | #![doc(test)]
    |  +
```