add regression test related to fix for JDK-8078024

Fix for JDK-8078024 may provoke previously rejected code to be accepted now. As an example this code:

import java.util.Arrays;
import java.util.List;

class K {
  public static <A> List<List<A>> cartesianProduct(List<? extends A>... lists) {
    return cartesianProduct(Arrays.asList(lists));
  }

  public static <B> List<List<B>> cartesianProduct(List<? extends List<? extends B>> lists) {
    throw new AssertionError();
  }
}

which was rejected before the patch, is now accepted by javac. See original report, by Liam Miller-Cushon, and discussion at: http://mail.openjdk.java.net/pipermail/compiler-dev/2015-July/009655.html

A regression test based on this code should be added to javac's test suite.