Work towards https://github.com/flutter/flutter/issues/142178.

---

This PR makes no _behavioral_ changes to executed code, and instead
focuses on organization and naming:

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)