Refactor and split test_service into test_{service,client}

[christophebedard]

The service/client instrumentation isn't currently complete (see #143), so the test_service tracing test wasn't really maintained. To be more in line with other tests:

1. Modify the test_service_ping and test_service_pong test nodes to have only 1 client in the test_sevice_ping node and only 1 service in the test_service_pong node. The goal is simply to get a ping->pong communication and then shut down. Therefore, unlike pubs/subs, we only need 1 pair of client/service: we can simply do a request->response. Also, simplify the code a bit.
2. Change test_service.py to only assert service-related events; nothing client-related. This is more in line with test_publisher.py/test_subscription.py.
3. Add test_client.py and only assert client-related events; nothing service-related. This means that the test is quite simple.

Requires ros2/rclcpp#2670 to fix a missing tracepoint trigger.

---

action-ros-ci-repos-override: https://gist.githubusercontent.com/christophebedard/49dc900164d67c8bc994a9104e7fd0b1/raw/fb5b5ef110c38880fd5ce71a21102e0776343058/ros2.repos

[clalancette]

Looks reasonable to me with green CI.

[christophebedard]

Pulls: #144, ros2/rclcpp#2670
Gist: https://gist.githubusercontent.com/christophebedard/b3572b9fa2f4051bb29d608e9ea6d45a/raw/7344d55e9c011cd93cf654afa10ea4e7c889938c/ros2.repos
BUILD args: --packages-above-and-dependencies rclcpp test_tracetools
TEST args: --packages-above rclcpp test_tracetools
ROS Distro: rolling
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/14807

• Linux
• Linux-aarch64
• Linux-rhel
• Windows

[fujitatomoya]

lgtm with green CI