Use raw string in paths in webidl-tests generated code

This fixes things on Windows, which uses backslashes in their paths

crates/webidl-tests/build.rs

@@ -28,7 +28,7 @@ fn main() {
                 use wasm_bindgen::prelude::*;
                 use wasm_bindgen_test::*;
 
-                #[wasm_bindgen(module = "{}")]
+                #[wasm_bindgen(module = r"{}")]
                 extern {{
                     fn not_actually_a_function{1}();
                 }}