Add Testing Guidelines

[maxime-clem]

The testing guidelines (https://autowarefoundation.github.io/autoware-documentation/main/contributing/testing-guidelines/) are currently empty.

Possible guidelines:

• Minimum line coverage (e.g., 90%).
• Tests structure (subfolder test, added with cmake command ament_add_ros_isolated_gtest, ...).
• Guideline for testing a ROS node (in gtest using rclcpp ? with pytest ?).
• ...

This discussion may be related to autowarefoundation/autoware-documentation#2

[maxime-clem]

I have opened a first PR to add guidelines for unit testing and integration testing: autowarefoundation/autoware-documentation#119
These guidelines are mostly copied from Autoware.Auto (https://autowarefoundation.gitlab.io/autoware.auto/AutowareAuto/testing-overview.html).