Runs a test app and a driver test simultaneously, then stops the app.
Usage:
```
flutter drive --target=/path/to/test/app.dart
```
This command will look for `/path/to/test/app_test.dart` by
convention. We will expand into other ways of discovering tests in the
future.
This commit contains:
- FlutterDriver API for e2e tests usable in conjunction with package:test
- FlutterDriverExtension to be enabled by the application in order to
allow an external agent to connect to it and drive user interactions and
probe into the element tree
- initial implementations of tap, findByValueKey and getText commands (to
be expanded in future PRs)
