(core) Fix nbrowser/CopyPaste test on Mac

Summary: `nbrowser/CopyPaste` was failing on Mac. Diff fixes that issue.

Test Plan: Update `test/nbrowser/CopyPaste` and `core/test/server/customUtil.ts`.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3552

test/server/customUtil.ts

@@ -54,8 +54,8 @@ export async function serveSomething(setup: (app: express.Express) => void, port
   });
 
   async function shutdown() {
-    await fromCallback(cb => server.close(cb));
     for (const conn of connections) { conn.destroy(); }
+    await fromCallback(cb => server.close(cb));
   }
 
   port = (server.address() as AddressInfo).port;