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)?