Test suite does not compile: binary operation `==` cannot be applied to type `syntax::ast::Crate`

As of rustc 1.29.0-nightly (31f1bc7b4 2018-07-15):

```console
error[E0369]: binary operation `==` cannot be applied to type `syntax::ast::Crate`
   --> tests/test_round_trip.rs:113:24
    |
113 |                     if before == after {
    |                        ^^^^^^^^^^^^^^^
    |
    = note: an implementation of `std::cmp::PartialEq` might be missing for `syntax::ast::Crate`
```