Auto merge of rust-lang#626 - fitzgen:sanity-test-stylo, r=emilio

Add a Stylo bindings sanity test

This commit adds a sanity test that we can generate bindings for Stylo without
any errors. I tried to make this a `#[bench]` but each iteration takes 36
seconds on my machine, which made the `#[bench]` take *way* too long. Instead,
there is a commented out `panic!` that can be uncommented to get a log of how
long it took.

r? @emilio

Author: bors-servo

.gitattributes

@@ -0,0 +1 @@
+tests/stylo.hpp -diff

Cargo.toml

@@ -16,7 +16,11 @@ documentation = "https://docs.rs/bindgen"
 version = "0.23.0"
 build = "build.rs"
 
-exclude = ["tests/headers", "tests/expectations", "bindgen-integration", "ci"]
+exclude = [
+  "bindgen-integration",
+  "ci",
+  "tests/**",
+]
 
 [badges]
 travis-ci = { repository = "servo/rust-bindgen" }