[FR]: Migrate project to testFixtures

[SimonMarquis]

Is there an existing issue for this?

• I have searched the existing issues

Describe the problem

Even though test fixtures are not completely supported by the Android Gradle Plugin (specifically with Kotlin sources) we can still benefit a lot from this feature on JVM modules, and this should definitely be part of Android dev's best practices!

Describe the solution

This would remove a lot of code duplication and prevent unecessary hacks like shared sourcesSet and gradle fixtures modules that could impact build times, especially in large code bases.
In the current state of the code base, it sheds ~1000 lines of code.

At my current company, we chose to use kotlin.Random as receiver for these testFixtures to make them more explicit about their behavior (being random by default), but this can be changed here if needed.

Next steps will be to migrate:

• "test" modules: :core:data-test, :core:datastore-test, :sync-test
• fakes and test classes like FakeStringDecoder, TestAuthorsRepository, TestNewsRepository, TestTopicsRepository, TestUserDataRepository

These next steps could already be done, but would need to rewrite classes in Java to be visible, and I don't think this is a path we want to go for this project.
So I guess we'll wait for the full Android/Kotlin support.

Additional context

No response

Code of Conduct

• I agree to follow this project's Code of Conduct

[JolandaVerhoef]

This looks like an interesting addition, however I'd like to wait for support by Jetbrains (through https://youtrack.jetbrains.com/issue/KT-50667) before adding this to the project.

[SimonMarquis]

Just to clarify, testFixture are officially supported.
The only bit that is not yet supported is exposing Kotlin sources from an AGP (com.android.application or com.android.library) module.
But I can understand if we don't want to include this feature until it is completely supported 👍

[yogurtearl]

Blocked by this issue: https://issuetracker.google.com/issues/259523353

[SimonMarquis]

For anyone interested in updates, I've pushed a new PR that enables "almost" proper Android test fixtures thanks to AGP 8.5.0:

• TestFixtures refactoring #1515