Wait for Worker in test to initialize before terminating

This fixes a sporadic panic caused by a race condition between a Worker
loading our test-specific Rust extension and the worker being shut down.
This shouldn't panic in principle, but fixing it would require
introducing a leak (see 2bf1525 for the change we could have made and
decided not to).

test/napi/lib/workers.js

@@ -4,6 +4,7 @@ const {
   isMainThread,
   parentPort,
   threadId,
+  workerData,
 } = require("worker_threads");
 
 const addon = require("..");
@@ -19,6 +20,7 @@ if (!isMainThread) {
   let boxed_channels = addon.box_channels();
 
   addon.get_or_init_thread_id(threadId);
+
   parentPort.once("message", (message) => {
     try {
       switch (message) {
@@ -47,6 +49,10 @@ if (!isMainThread) {
     }
   });
 
+  if (workerData === "notify_when_startup_complete") {
+    parentPort.postMessage("startup_complete");
+  }
+
   return;
 }
 
@@ -197,9 +203,13 @@ describe("Instance-local storage", () => {
   });
 
   it("should be able to exit a worker without a crash", (cb) => {
-    const worker = new Worker(__filename);
+    const worker = new Worker(__filename, {
+      workerData: "notify_when_startup_complete",
+    });
 
-    setTimeout(() => worker.terminate(), 50);
-    setTimeout(cb, 100);
+    worker.once("message", async () => {
+      await worker.terminate();
+      cb();
+    });
   });
 });