The ClassForNameLeak test includes a loop[1] that calls System.gc() in order to give some phantom references (to class loaders created earlier in the test) a chance to be enqueued for processing. When running on GraalVM, this loop sometimes does not run long enough to allow all phantom references to be enqueued, resulting in:

java.lang.RuntimeException: ClassLoader was never enqueued!
        at ClassForNameLeak.main(ClassForNameLeak.java:118)
        at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
        at java.base/java.lang.reflect.Method.invoke(Method.java:578)
        at com.sun.javatest.regtest.agent.MainWrapper$MainTask.run(MainWrapper.java:138)
        at java.base/java.lang.Thread.run(Thread.java:1583)

Bumping the number of loop iterations from 10 to 100 seems to reliably prevent the failure.

[1] https://github.com/openjdk/jdk/blob/09514745fc87d5a212bdf09399aa2045f02b4612/test/jdk/java/lang/ClassLoader/forNameLeak/ClassForNameLeak.java#L109-L112