Improve testing of parallel loading of shared classes by the boot class loader

This test should be improved to also test the loading of shared classes by the boot class loader
 
http://hg.openjdk.java.net/jdk/jdk/file/0d97bf7cf8a4/test/hotspot/jtreg/runtime/cds/appcds/ParallelLoadTest.java

In addition, asserts should be added to the JVM to verify that for a shared classes loaded the boot class loader, SystemDictionary::load_shared_class is not called multiple times.