Single instance: the stale-endpoint test connects to no freed port

The test bound a port, freed it and then connected to it as a gone app.
Tests run in parallel, so another test's listener could take that port
meanwhile and accept this connection in place of its own client's,
whose connection was then reset when the listener closed: the cause of
the intermittent tcp_connection_is_closed_after_junk failures on
Windows. The gone app is now a listener that closes without a greeting.

apps/vectorcraft/src/single_instance.rs

@@ -329,10 +329,15 @@ mod tests {
     #[test]
     fn a_stale_endpoint_file_is_ignored_and_replaced() {
         let (path, dir, file) = folder("stale");
-        // No listener on the recorded port any more.
-        let port = TcpListener::bind((Ipv4Addr::LOCALHOST, 0)).unwrap().local_addr().unwrap().port();
+        // The recorded app is gone: whatever holds the port closes without a greeting. (A freed
+        // port can be taken by another test's listener meanwhile, which would then accept this
+        // connection in place of its own client's.)
+        let gone = TcpListener::bind((Ipv4Addr::LOCALHOST, 0)).unwrap();
+        let port = gone.local_addr().unwrap().port();
         write_endpoint(&path, &json!({ "port": port, "token": "t", "hello": "h" })).unwrap();
+        let closed = std::thread::spawn(move || drop(gone.accept()));
         assert!(forward(&path, std::slice::from_ref(&file)).is_err());
+        closed.join().unwrap();
         // A listener that doesn't know the greeting gets neither the token nor the paths.
         let impostor = TcpListener::bind((Ipv4Addr::LOCALHOST, 0)).unwrap();
         let port = impostor.local_addr().unwrap().port();