This PR makes no _behavioral_ changes to executed code, and instead
focuses on organization and naming:
1. Almost[^1] anything named `external_ui` is renamed
`external_textures`
1. Extended the README to explain the intent of the test, as well as how
to run it
1. Renamed `main.dart` and `main_test.dart` to `frame_rate_main.dart`
and `frame_rate_test.dart` (we'll add more)
1. Did some refactoring of the test to make it more obvious what is
being asserted (i.e. `widgetBuilds` and friends)
Given how complex (and in-flux) this directory is, I'm also requesting
either John, Jonah or I review any changes.
[^1]: Except the name of the `.ci.yaml` task, i.e. `name:
Linux_pixel_7pro external_ui_integration_test` because I'm apparently
not able to change that without creating a new task as `bringup: true`
and playing a bit of a dance. Maybe that's worth doing though (in future
PRs)?
Remove self from `CODEOWNERS` file for `flutter_tools`. Now that we have the PR cocoon labeler the tools team should see any PRs that change these files, and tag the right people to review.
Also remove ownership of the `CODEOWNERS` file itself.
