Add tutorial on integration testing

[abaeyens]

This PR adds a tutorial on integration testing with launch_testing and complements the two existing tutorials on unit testing. This topic was first discussed in #4827 and thereafter on ROS discourse. This is my first PR for the ROS 2 project, all feedback welcome!

Resolves #4827 and closes ros2/launch#812.

[ros-discourse]

This pull request has been mentioned on ROS Discourse. There might be relevant details there:

https://discourse.ros.org/t/towards-a-rosdoc-integation-testing-tutorial/40701/2

[clalancette]

First of all, thank you for the tutorial. This is documentation we really need.

I've left quite a number of comments inline. Please take a look when you get a chance.

[clalancette]

Since this is a new tutorial that has nothing linking to it, we don't need this.

Suggested change

.. TestingIntegration:

[clalancette]

Suggested change

	Before starting this tutorial,
	it is recommended to have completed the following tutorials on launching nodes:
	Before starting this tutorial, it is recommended to have completed the following tutorials on launching nodes:

[clalancette]

Suggested change

	Where unit tests focus on validating a very specific piece of functionality,
	integration tests go all the way in the other direction.
	Where unit tests focus on validating a very specific piece of functionality, integration tests focus on validating the interaction between pieces of code.

[clalancette]

Suggested change

	In ROS 2 this means launching a system of one or several nodes,
	for example the Gazebo simulator and the Nav2 navigation stack.
	In ROS 2 this is often accomplished by launching a system of one or several nodes, for example the `Gazebo simulator <https://gazebosim.org/home>`__ and the `Nav2 navigation <https://github.com/ros-planning/navigation2.git>`__ stack.

[clalancette]

Suggested change

	A key aspect of ROS 2 integration testing is that nodes part of different tests
	shouldn't communicate with each other, even when run in parallel,
	which will be achieved here using a specific test runner
	that picks unique ROS domain id's.
	A key aspect of ROS 2 integration testing is that nodes that are part of different tests
	shouldn't communicate with each other, even when run in parallel.
	This will be achieved here using a specific test runner that picks unique :doc:`ROS domain IDs <../../../Concepts/Intermediate/About-Domain-ID>`.

[clalancette]

Suggested change

	* An undecorated class inheriting from ``unittest.TestCase``:
	houses the active tests, including set up and teardown.
	One has access to the ROS logging through ``proc_output``.
	* An undecorated class inheriting from ``unittest.TestCase``: This houses the active tests, including set up and teardown, and gives access to ROS logging through ``proc_output``.

[clalancette]

Suggested change

	* A second class inheriting from ``unittest.TestCase``,
	decorated with ``@launch_testing.post_shutdown_test()``.
	As the name implies, these tests run after all nodes have shutdown.
	A common assert here is to check the exit codes,
	to ensure all nodes exited cleanly.
	* A second class inheriting from ``unittest.TestCase`` decorated with ``@launch_testing.post_shutdown_test()``: These are tests that run after all nodes have shutdown; it is common to assert that the nodes exited cleanly.

[clalancette]

Suggested change

	The launch test is subsequently registered in the ``CMakeLists.txt``
	using the custom cmake macro ``add_ros_isolated_launch_test``
	that ensures that each launch test runs with a unique ``ROS_DOMAIN_ID``,
	avoiding undesired cross communication.
	The launch test is subsequently registered in the ``CMakeLists.txt`` using the custom cmake macro ``add_ros_isolated_launch_test`` which ensures that each launch test runs with a unique ``ROS_DOMAIN_ID``,
	avoiding undesired cross communication.

[clalancette]

Suggested change

	To finish, tools such as Xunit Viewer ease visualizing the test results
	in a more colorful way than the colcon utilities.
	We hope this tutorial gave the reader a good grasp
	of how to conduct integration tests in ROS 2,
	delineating tests, and analyzing their results.

[clalancette]

This is basically "homework", and while I don't necessarily disagree with it, this isn't what we do in our other tutorials. I would suggest just removing this for now.