Fix permission_prompt_test (#1509)

tools/permission_prompt_test.ts

@@ -11,8 +11,15 @@ const test = {
   needsNet: () => {
     listen("tcp", "127.0.0.1:4540");
   },
-  needsRun: () => {
-    run({ args: ["python", "-c", "import sys; sys.stdout.write('hello')"] });
+  needsRun: async () => {
+    const process = run({
+      args: [
+        "python",
+        "-c",
+        "import sys; sys.stdout.write('hello'); sys.stdout.flush()"
+      ]
+    });
+    await process.status();
   }
 }[name];