cmd/swarm: solve rare cases of using the same random port in tests (#17352)

cmd/swarm/config_test.go

@@ -20,6 +20,7 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
+	"net"
 	"os"
 	"os/exec"
 	"testing"
@@ -559,3 +560,16 @@ func TestValidateConfig(t *testing.T) {
 		}
 	}
 }
+
+func assignTCPPort() (string, error) {
+	l, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		return "", err
+	}
+	l.Close()
+	_, port, err := net.SplitHostPort(l.Addr().String())
+	if err != nil {
+		return "", err
+	}
+	return port, nil
+}