Generate code in test sources

[edwardharks]

At the moment the code is generated in the main directory. It should be generated in the test source directory.

[edwardharks]

google/ksp#799 (comment)

[edwardharks]

Potential alternative approach

// class in main sources that we want to generate a fixture for
class User

// object annotated with @Fixture and that class that fixture is for in test sources
@Fixture(User::class)
object UserFixtures

// Generate an extension function for creating the fixture in test sources
fun UserFixtures.user(): User = User()