Move bindings tests out of the style crate.

This cuts in almost half the time to run:

```
touch components/style/lib.rs
./mach test-stylo
```

tests/unit/stylo/build.rs

@@ -2,6 +2,10 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+use std::env;
+use std::fs;
+use std::io::Write;
+use std::path;
 use std::process::Command;
 
 fn main() {
@@ -11,4 +15,13 @@ fn main() {
     println!("cargo:rerun-if-changed=../../../components/style/gecko_bindings/bindings.rs");
     assert!(Command::new("python").arg("./check_bindings.py")
                                   .spawn().unwrap().wait().unwrap().success());
+
+    // https://github.com/rust-lang/cargo/issues/3544
+    let style_out_dir = env::var_os("DEP_FOR SOME REASON THE LINKS KEY IS REQUIRED \
+                                     TO PASS DATA AROUND BETWEEN BUILD SCRIPTS_OUT_DIR").unwrap();
+    fs::File::create(path::PathBuf::from(env::var_os("OUT_DIR").unwrap()).join("bindings.rs"))
+        .unwrap()
+        .write_all(format!("include!(concat!({:?}, \"/gecko/structs_debug.rs\"));",
+                           style_out_dir).as_bytes())
+        .unwrap();
 }

tests/unit/stylo/lib.rs

@@ -17,3 +17,9 @@ mod size_of;
 
 mod servo_function_signatures;
 
+use style::*;
+
+#[allow(dead_code, improper_ctypes)]
+mod bindings {
+    include!(concat!(env!("OUT_DIR"), "/bindings.rs"));
+}