RFR: 8375485: Tests in vmTestbase/nsk are failing due to missing class unloading after 8373945

[SendaoYan]

Hi all,

This PR make 2 changes which make test more rubustness:

1. Add max 10 times tries which removed by JDK-8342659, this will make test more steady after JDK-8373945.
2. test/hotspot/jtreg/vmTestbase/nsk/jvmti/scenarios/events/EM02/em02t003.java try to invoke thread and do not wailt the thread run finish unload the class immediately. This PR wait util the thread is non alive, and wait more 2 senonds to make sure the thread exit completely. This will make test more robustness, before this PR vmTestbase/nsk/jvmti/scenarios/events/EM02/em02t003/TestDescription.java fails probability 1/50, after this PR test run all passed 10k times.

Additional testing:

- [ ] All tests touched test by JDK-8373945

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Commit messages:
 - 8375485: Tests in vmTestbase/nsk are failing due to missing class unloading after JDK-8373945

Changes: https://git.openjdk.org/jdk/pull/29285/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29285&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375485
  Stats: 21 lines in 2 files changed: 20 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/29285.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29285/head:pull/29285

PR: https://git.openjdk.org/jdk/pull/29285