Address review feedback.

Specifically:
  1. Move the net package to util/testhelpers.
  2. Remove the concurrency test.
  3. Rename the SignerServer listner member.

arbnode/dataposter/externalsignertest/externalsignertest.go

@@ -22,8 +22,7 @@ import (
 	"github.com/ethereum/go-ethereum/common/hexutil"
 	"github.com/ethereum/go-ethereum/rpc"
 	"github.com/offchainlabs/nitro/arbnode/dataposter/externalsigner"
-
-	nnet "github.com/offchainlabs/nitro/net"
+	"github.com/offchainlabs/nitro/util/testhelpers"
 )
 
 var (
@@ -42,7 +41,7 @@ type CertAbsPaths struct {
 type SignerServer struct {
 	*http.Server
 	*SignerAPI
-	l net.Listener
+	listener net.Listener
 }
 
 func basePath() (string, error) {
@@ -101,7 +100,7 @@ func NewServer(t *testing.T) *SignerServer {
 	pool := x509.NewCertPool()
 	pool.AppendCertsFromPEM(clientCert)
 
-	ln, err := nnet.FreeTCPPortListener()
+	ln, err := testhelpers.FreeTCPPortListener()
 	if err != nil {
 		t.Fatalf("Error getting a listener on a free TCP port: %v", err)
 	}
@@ -147,7 +146,7 @@ func (s *SignerServer) Start() error {
 	if err != nil {
 		return err
 	}
-	if err := s.ServeTLS(s.l, cp.ServerCert, cp.ServerKey); err != nil && !errors.Is(err, http.ErrServerClosed) {
+	if err := s.ServeTLS(s.listener, cp.ServerCert, cp.ServerKey); err != nil && !errors.Is(err, http.ErrServerClosed) {
 		return err
 	}
 	return nil

net/port.go → util/testhelpers/port.go

@@ -1,5 +1,4 @@
-// Package net provides facilities for working with network connections.
-package net
+package testhelpers
 
 import (
 	"net"

util/testhelpers/port_test.go

@@ -0,0 +1,23 @@
+package testhelpers
+
+import (
+	"net"
+	"testing"
+)
+
+func TestFreeTCPPortListener(t *testing.T) {
+	aListener, err := FreeTCPPortListener()
+	if err != nil {
+		t.Fatal(err)
+	}
+	bListener, err := FreeTCPPortListener()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if aListener.Addr().(*net.TCPAddr).Port == bListener.Addr().(*net.TCPAddr).Port {
+		t.Errorf("FreeTCPPortListener() got same port: %v, %v", aListener, bListener)
+	}
+	if aListener.Addr().(*net.TCPAddr).Port == 0 || bListener.Addr().(*net.TCPAddr).Port == 0 {
+		t.Errorf("FreeTCPPortListener() got port 0")
+	}
+}