Fixed update period computation in test (#693)

(cherry picked from commit de7083e)

joint_trajectory_controller/test/test_trajectory_controller_utils.hpp

@@ -386,9 +386,13 @@ class TrajectoryControllerTest : public ::testing::Test
     auto clock = rclcpp::Clock(RCL_STEADY_TIME);
     const auto start_time = clock.now();
     const auto end_time = start_time + wait_time;
+    auto previous_time = start_time;
+
     while (clock.now() < end_time)
     {
-      traj_controller_->update(clock.now(), clock.now() - start_time);
+      auto now = clock.now();
+      traj_controller_->update(now, now - previous_time);
+      previous_time = now;
     }
   }