I can do this.

kotlin {
    jvm()
    js {
        nodejs {
            testTask {
                debug true
            }
        }
    }

This starts the Node.js debugger, but stops Gradle task execution because it starts with

--inspect-brk

as in here https://github.com/JetBrains/kotlin/blob/8995d89599690867782ed43e5a9bce47016bd2eb/libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/targets/js/testing/KotlinJsTest.kt#L108 I can connect other debugger such as Chrome or IntelliJ Node.js debug, but it won't go through the Kotlin code.

update: it will go through Kotlin code, but only if I set breakpoint in JavaScript code and step in the Kotlin code