[11] RFR JDK-8168682: jdk/test/java/lang/ClassLoader/forNameLeak/ClassForNameLeak.java fails with -Xcomp

[mandy chung]

This patch revises the test to make the phantom reference reachable
in order to ensure that the reference is enqueued for verification.

Webrev at:
http://cr.openjdk.java.net/~mchung/jdk11/webrevs/8168682/webrev.00/

thanks
Mandy