RFR(S) 8231125 Improve testing of parallel loading of shared classes by the boot class loader

[Ioi Lam]

https://bugs.openjdk.java.net/browse/JDK-8231125
http://cr.openjdk.java.net/~iklam/jdk14/8231125-shared-boot-class-parallel-test.v01/


[1] Modified ParallelLoadTest.java to also test the loading of shared 
classes
     into the boot loader.
[2] Modified ParallelLoad.java to minimize the initial time lag when the 
4 parallel
     threads compete to load the same class.
[3] Added an assert in the JVM to verify that for a shared classes loaded
     the boot class loader, SystemDictionary::load_shared_class is not 
called
     multiple times.