The AntBuilder classpath error was caused from using GradleRunner.withDebug(true) - it is a known issue and it looks like it is not easy to address.
Next problem was that the worker fails with NPE when any of the WorkerConfiguration::params is null - missing null check in the code determining the argument types.
I had to introduce a dummy default value and then check for it, rather than relying on the Kotlin’s ?: construct…