test: add integration tests running inside a Flutter app

[EchoEllet]

Originally, I wrote these integration tests to verify #15 (the updated simple_secure_storage_linux implementation), since they can run directly using Dart without an example app or a Flutter engine, as the updated implementation does not depend on Flutter method channels.

However, that skips the platform implementation registration path, and the tests are verifying the public behavior of SimpleSecureStorage so they can be reused on other platforms as well. So I kept them in simple_secure_storage/example/integration_test.

I ran the tests on CachyOS (KDE Plasma, KWallet) using: flutter test integration_test/secure_storage_test.dart -d linux (inside simple_secure_storage/example).

These tests do not test everything or platform-specific edge cases; however, freedesktop_secret already has integration tests.

Note: These tests share similarities with some of the packages in this repository.