Update test/fetch/cookies.js

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: Uzlopak

test/fetch/cookies.js

@@ -109,6 +109,7 @@ describe('cookies', () => {
     strictEqual(await response.text(), 'test')
 
     await client.close()
-    await new Promise((resolve, reject) => server.close(err => err ? reject(err) : resolve()))
+    server.close()
+    await once(server, 'close')
   })
 })