Depend on project as a test project?

I have a project that contains shared code that is only used for testing purposes. But how can I tell my other projects to depend on it as a test dependency? Doing something like this will make it a compile-time dependency for a and b:

```kotlin
val testingProject = project { .. } 
val a = project(testingProject) { .. } 
val b = project(testingProject) { .. } 
```