abstract fun <T : Spec> instantiate(clazz: KClass<T>): Spec?
This function is invoked to create an instance of a Spec.
By default, test classes are assumed to have zero-arg primary constructors. If you wish to use non-zero arg primary constructors, this function can be implemented with logic on how to instantiate a test class.
An implementation can choose to create a new instance, or it can choose to return null if it wishes to pass control to the next extension (or if no more extensions, then back to the Engine).
By overriding this function, extensions are able to customize the way classes are created, to support things like constructors with parameters, or classes that require special initization logic.
One very common usecase is to instantiate classes that are being autowired by a dependency injection framework.