Test file location wasn't clear, added example file tree

[Jakeisthesnake]

[I am a beginner with ROS, but from what I could gather by looking at other repositories, the test files go in the root of the package directory. Please reject this pull request if that isn't accurate. Or leave feedback and I can correct it.]

I also changed the names to match the example file tree.

One example of where test files are stored in another repository. https://github.com/ros2/geometry2/tree/rolling/tf2