restore rclcpp:shutdown()

Without the shutdown: The tests are passing in `RUN_ALL_TESTS()`, the test itself is failing with timeout. Most probably because `server_thread.join()` hangs forever.

opennav_coverage_bt/test/test_compute_coverage_path.cpp

@@ -169,7 +169,7 @@ int main(int argc, char ** argv)
   int all_successful = RUN_ALL_TESTS();
 
   // shutdown ROS
-  // rclcpp::shutdown();
+  rclcpp::shutdown();
   server_thread.join();
 
   return all_successful;