I would like to extend run_test.py to provide ROS_DOMAIN_ID isolation

[pbaughman]

There's a function in launch_testing that picks a ROS_DOMAIN_ID between 1 and 100 that's unique on a given machine that's used to run launch_tests in isolation.

I'd like to use the same mechanism to allow ament_cmake_test's run_test.py to provide isolation for gtests and pytests. run_test.py. I'd like to accomplish this by doing the following:

1. Refactor the domain_coordinator into its own python package so launch_testing and ament_cmake_test can both use it.
2. Change run_test.py to use the same logic as launch_test. If no ROS_DOMAIN_ID is set, set a unique one in the environment of the test. If ROS_DOMAIN_ID is already set, we don't do anything - just use that same ROS_DOMAIN_ID
3. Plumb an argument to ament_add_gtest and ament_add_pytest down into ament_add_test to disable the isolation behavior. This argument matches the one from launch_testing

Before I start, I want to find out if anybody has strong opinions about this. @tfoote first mentioned solving this problem here ros2/launch#251 (review)

[pbaughman]

I would also like to avoid a repeat of ros2/launch#253.

Perhaps run_test.py could load an extension provided by another package, and ament_add_gtest and ament_add_pytest could be extended to take arbitrary arguments? That way run_test.py doesn't need to know about ROS_DOMAIN_ID and we can provide a very thin wrapper that turns on isolation instead of having to replicate all the functionality.

[pbaughman]

@wjwwood has made some suggestions in an IRL conversation - I will summarize my approach below:

1. Refactor domain coordinator into its own python package so it can be used for this, and by launch_testing_ros
2. Add an argument to ament_add_test that lets you optionally specify a different test runner and plumb this argument up to ament_add_gtest and ament_add_pytest
3. Refactor run_test.py so its core functionality is in a module that can be imported
4. Add a new package with a new test runner uses the domain coordinator and sets ROS_DOMAIN_ID. This package will provide ament_add_isolated_gtest and ament_add_isolated_pytest which work exactly like ament_add_gtest and ament_add_pytest, but specify the new test runner instead of the original run_test.py

Items 2 and 3 will be done on this repo. Items 1 and 4 would be a better fit for https://github.com/ros2/ament_cmake_ros

[pbaughman]

Items 2 and 3 are in this PR: #174
Items 1 and 4 are in this PR: ros2/ament_cmake_ros#3