function fetch_tests_from_shadow_realm(realm) {
        var chan = new MessageChannel();
        function receiveMessage(msg_json) {
            chan.port1.postMessage(JSON.parse(msg_json));
        }
        var done = tests.fetch_tests_from_worker(chan.port2);
        realm.evaluate("begin_shadow_realm_tests")(receiveMessage);
        chan.port2.start();
        return done;
    }
Get test results from a shadow realm and include them in the current test.

@param {ShadowRealm} realm - A shadow realm also running the test harness
@returns {Promise} - A promise that's resolved once all the remote tests are complete.