Groovy with @Grab annotation lets JUnit fail (No suitable ClassLoader found for grab)

This simple test leads to: java.lang.ExceptionInInitializerError ....
Caused by: java.lang.RuntimeException: No suitable ClassLoader found for grab ...

//--- production ---
package mytest

@Grab(group='commons-io', module='commons-io', version='2.4')
class MyTest {
}

// --- Tests ----
package mytest

import org.junit.*

class MyTestTest {

 @Test
 void 'should not fail'()
 {
 def t = new MyTest()
 }
}

Has anybody an idea how to get this running in IntelliJ IDEA 16.2

Regards,
Gert