zjsunit: Support asynchronous overrides.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

frontend_tests/zjsunit/namespace.js

@@ -446,15 +446,34 @@ exports.with_overrides = function (test_function) {
     };
 
     let ret;
+    let is_promise = false;
     try {
         ret = test_function({override, override_rewire, disallow, disallow_rewire});
+        is_promise = typeof ret?.then === "function";
+        ok = !is_promise;
+    } finally {
+        if (!is_promise) {
+            restore_callbacks.reverse();
+            for (const restore_callback of restore_callbacks) {
+                restore_callback();
+            }
+        }
+    }
+
+    if (!is_promise) {
+        return ret;
+    }
+
+    return (async () => {
+        try {
+            ret = await ret;
             ok = true;
+            return ret;
         } finally {
             restore_callbacks.reverse();
             for (const restore_callback of restore_callbacks) {
                 restore_callback();
             }
         }
-
+    })();
-    return ret;
 };