Cannot compile test programs using IDEA Kotlin multiplatform (experimental)

I'm evaluating Kotlin on IDEA, using the multiplatform option, which is marked as experimental (and using Gradle). I can develop and run shared, JVM and JS code OK, but my problem is compiling test programs for execution under TestNG.

Running the :compileTestKotlinCommon task, both from IDEA and from a console, it clears and re-generates the expected output directory (build/classes/kotlin/test/..., but only puts kotlin_metadata files in there.

Before I get into posting configuation details, does anyone know if the 'Experimental' tag to the IDEA option means I'm breaking new ground here, and what I'm attempting may not even work yet?

I have a feeling that there should be some JVM class files being generated from the Kotlin test classes, which the test harness could then execute, but can't see how the Gradle config generated by IDEA could possibly generate these.

Should I keep trying, or just find a work-around?