The README section for Fall-back to classpath mode states that using the Kotlin DSL, you can enable classpath mode while running tests with:

tasks {
  compileTestJava {
        extensions.configure(CompileTestModuleOptions::class) {
            compileOnClasspath = true
        }
    }
}

In fact, in version 1.8.15 of the plugin, you have to write isCompileOnClasspath = true. The reason for this is that the accessor needs to be called getCompileOnClasspath() for it to be treated as a property in Kotlin.

Note that this doesn't affect the Groovy DSL, and that TestModuleOptions works as documented for both Kotlin and Groovy.

I've adjusted the tests to demonstrate the problem, implemented a fix, and deprecated the old method.

Let me know if you want to take a different approach, or you'd like anything adjusting.