Enable unstable features of rustfmt when running tests

Author: nox

tests/tests.rs

@@ -47,6 +47,7 @@ The latest `rustfmt` is required to run the `bindgen` test suite. Install
             "--config-path",
             concat!(env!("CARGO_MANIFEST_DIR"), "/tests/rustfmt.toml")
         ])
+        .env("CFG_RELEASE_CHANNEL", "nightly")
         .stdin(process::Stdio::piped())
         .stdout(process::Stdio::piped())
         .stderr(process::Stdio::piped())