test(worker): make the message flood test deterministic instead of racing worker boot

test/js/web/workers/worker.test.ts

@@ -412,11 +412,16 @@ describe("web worker", () => {
       const src = `const p = { s: Buffer.alloc(200, "x").toString(), a: [1, 2, 3], n: 0 };
         (function burst() { for (let i = 0; i < 2000; i++) { p.n++; postMessage(p) } setImmediate(burst) })()`;
       const w = new Worker(URL.createObjectURL(new Blob([src])));
-      let received = 0;
-      w.onmessage = () => received++;
+      // Booting the worker is not the property under test (on a debug build it takes
+      // far longer than the timer turns below): start once the flood has reached the parent.
+      await once(w, "message");
       // Three timer turns while the flood is running is the property; not the timing.
-      for (let i = 0; i < 3; i++) await new Promise<void>(r => setTimeout(r, 10));
-      expect(received).toBeGreaterThan(0);
+      // Pinned inside one drain, the parent never gets to the timer; the message awaited
+      // after each turn shows the flood was still running while the turn was taken.
+      for (let i = 0; i < 3; i++) {
+        await new Promise<void>(r => setTimeout(r, 10));
+        await once(w, "message");
+      }
       w.terminate();
       await once(w, "close");
     });