RFR: 8376023: Reconcile ClassUnloader with ClassUnloadCommon

[SendaoYan]

Hi all,

This PR merge test/hotspot/jtreg/vmTestbase/nsk/share/ClassUnloader.java to test/lib/jdk/test/lib/classloader/ClassUnloadCommon.java, and make the vmTestbase tests use the ClassUnloadCommon.

Additional testing:

- [ ] Full jtreg tests include vmTestbase

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Commit messages:
 - Update comment
 - Remove test/hotspot/jtreg/vmTestbase/nsk/share/ClassUnloader.bak
 - Add "import jdk.test.lib.classloader.ClassUnloadCommon;"
 - Add "import jdk.test.lib.classloader.ClassUnloadCommon;"
 - Replace ClassUnloader as ClassUnloadCommon
 - Fix test/hotspot/jtreg/vmTestbase/nsk/jdi/ClassUnloadRequest/addClassFilter/filter001/TestDescription.java
 - Fix vmTestbase/nsk/jvmti/scenarios/events/EM07/em07t002/TestDescription.java
 - Fix vmTestbase/nsk/jdi/ReferenceType/fields/fields003/TestDescription.java
 - Fix vmTestbase/nsk/jvmti/ObjectFree/objfree001/TestDescription.java
 - Fix vmTestbase/nsk/jdi/VirtualMachine/instanceCounts/instancecounts003/instancecounts003.java
 - ... and 3 more: https://git.openjdk.org/jdk/compare/e21cb852...458fb127

Changes: https://git.openjdk.org/jdk/pull/29545/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29545&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8376023
  Stats: 613 lines in 71 files changed: 244 ins; 275 del; 94 mod
  Patch: https://git.openjdk.org/jdk/pull/29545.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29545/head:pull/29545

PR: https://git.openjdk.org/jdk/pull/29545