RFR (S): 8242237: Improve JVM TI HiddenClasses tests

[serguei.spit...@oracle.com]

Please, review a fix for the sub-task:
  https://bugs.openjdk.java.net/browse/JDK-8242237

Webrev:
  http://cr.openjdk.java.net/~sspitsyn/webrevs/2020/jvmti-hidden-test-update.1/

Summary:
  The test update includes:
   - interface and method renaming: Test => HCInterf, test() => hcMethod()
   - readClassFile replaced with Files.readAllBytes
   - added ClassPrepare event
   - removed unneeded capability can_get_source_file_name
   - added more comments

Testing:
  Tested locally on Linux, mach5 test run is in progress:
   https://mach5.us.oracle.com/mdash/jobs/sspitsyn-HiddenClass-jvmti-test-20200424-1829-10485216

Thanks,
Serguei