Coverage

The `flutter test --coverage` run the tests and create a coverage report in `coverage/lcov.info` file.
However the coverage doesn't catch untested files which make it difficult to know the coverage for all the project.
A solution is to create a dummy test which import all the dart files of the Flutter project. This force the coverage to be updated for the project scope.

- [x] Create a bash script to generate the dummy test importing all the dart files
- [x] Run the script in Github actions before running the `flutter test --coverage` command

see
- https://stackoverflow.com/questions/54602840/how-can-i-generate-test-coverage-of-untested-files-on-my-flutter-tests
- https://github.com/flutter/flutter/issues/27997