Update JSON loader in search tests

Author: GuillaumeGomez

tests/rendered_output.rs

@@ -773,9 +773,10 @@ mod search {
     fn read_book_index(root: &Path) -> serde_json::Value {
         let index = root.join("book/searchindex.js");
         let index = fs::read_to_string(index).unwrap();
-        let index = index.trim_start_matches("Object.assign(window.search, ");
-        let index = index.trim_end_matches(");");
-        serde_json::from_str(index).unwrap()
+        let index = index.trim_start_matches("window.search = JSON.parse('");
+        let index = index.trim_end_matches("');");
+        // We need unescape the string as it's supposed to be an escaped JS string.
+        serde_json::from_str(&index.replace("\\'", "'").replace("\\\\", "\\")).unwrap()
     }
 
     #[test]