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You should use a CommandLineArgumentProvider with jvmArgumentProviders:

testTask.configure {
    jvmArgumentProviders.add(
        JavaAgentArgumentProvider(someConfig))
}

class JavaAgentArgumentProvider(
    @get:Input val javaagent: FileCollection
) : CommandLineArgumentProvider {
    override fun asArguments() =
        listOf("-javaagent:${javaagent.asPath}")
}