[iron] add test to check for execution order of entities in various executors
This is the Iron version of https://github.com/ros2/rclcpp/pull/2536 and passes without changes to the actual source of rclcpp, which supports the idea that this test is actually testing a regression between iron and jazzy. The fix for jazzy/rolling has been proposed here: https://github.com/ros2/rclcpp/pull/2537
Unfortunately the order of execution for timers seems to be subtly out of the expected order on Windows 🤦 . I'll look into it more tonight.
Since this is targeting Iron, which is going EOL soon, I'm going to close this out.
@wjwwood if you think it makes sense to retarget this to rolling, please feel free to reopen and retarget.