[Internal tests] Close MessageChannel port to prevent leak

Node's MessageChannel implementation will leak if you don't explicitly
close the port. This updates the enqueueTask function we use in our
internal testing helpers to close the port once it's no longer needed.

packages/internal-test-utils/enqueueTask.js

@@ -11,6 +11,9 @@ const {MessageChannel} = require('node:worker_threads');
 
 export default function enqueueTask(task: () => void): void {
   const channel = new MessageChannel();
-  channel.port1.onmessage = task;
+  channel.port1.onmessage = () => {
+    channel.port1.close();
+    task();
+  };
   channel.port2.postMessage(undefined);
 }