Migrate `validate_json.py` script to rust in `run-make/rustdoc-map-file` test This PR fixes the FIXME I added for future-me who become present-me. :') Since there are multiple `run-make` tests using python scripts, I suppose more of them will migrate to Rust, hence why I added the `jzon` public reexport to the `run-make-support` crate. cc `@jieyouxu` r? `@Kobzol`