[TESTBUG] TestHumongousClassLoader.java can fail on systems with non-g1 collector.

gc/g1/humongousObjects/TestHumongousClassLoader executes gc.g1.humongousObjects.ClassLoaderGenerator without specifying the GC:
 
 * @run main/othervm/timeout=240 -Xms256M -Xmx256M -XX:+UnlockDiagnosticVMOptions -XX:+WhiteBoxAPI -Xbootclasspath/a:.
 * gc.g1.humongousObjects.ClassLoaderGenerator 1

However ClassLoaderGenerator instantiate G1SampleClass which has G1 specific WhiteBox method. It may cause to failure on system with non-g1 collector. Need to add -XX:+UseG1GC.