Set up testing and code coverage reports

[Codel1417]

Codecov token already set up. Need to create test action which sets up the environment and runs flutter test

- name: Upload coverage reports to Codecov
    uses: codecov/codecov-action@v4.0.1
    with:
      token: ${{ secrets.CODECOV_TOKEN }}

• Create testing job
• Configure basic unit testing in flutter
• achieve 10% code coverage
• achieve 20% code coverage

The end goal would be 100% coverage, but 10% is just a start.

https://docs.flutter.dev/testing/overview
https://app.codecov.io/gh/OpenTails/tail_app/new