Fix: do not hardcode test port

psiphon/common/transforms/httpTransformer_test.go

@@ -386,13 +386,18 @@ func TestHTTPTransformerHTTPServer(t *testing.T) {
 				}()
 			})
 
+			listener, err := net.Listen("tcp", "127.0.0.1:0")
+			if err != nil {
+				t.Fatalf("net.Listen failed %v", err)
+			}
+
 			s := &http.Server{
-				Addr:    "127.0.0.1:8080",
+				Addr:    listener.Addr().String(),
 				Handler: mux,
 			}
 
 			go func() {
-				s.ListenAndServe()
+				s.Serve(listener)
 			}()
 
 			client := http.Client{