RFR: 8373945: vmTestbase eatMemory/ClassUnloader provoke OOME to force GC and might cause GC in other threads

[SendaoYan]

Hi all,

This PR use `WhiteBox.getWhiteBox().fullGC()` instead of `eatMemory` to grigger full GC. The OOME trigger by `eatMemory` may cause vmTestbase/nsk/monitoring/stress/classload tests intermittent fails when run those tests simultancely on some machines. The WB.fullGC() might be use for same purpose. It also reduce test execution time.

Change has been verified locally by running tests vmTestbase/nsk/monitoring/stress/classload on linux-x64.

Additional testing:

- [ ] All jtreg tests by fastdebug build

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Commit messages:
 - Remove unnecessary unloadClass
 - 8373945: vmTestbase eatMemory/ClassUnloader provoke OOME to force GC and might cause GC in other threads

Changes: https://git.openjdk.org/jdk/pull/28891/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28891&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373945
  Stats: 157 lines in 25 files changed: 73 ins; 54 del; 30 mod
  Patch: https://git.openjdk.org/jdk/pull/28891.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28891/head:pull/28891

PR: https://git.openjdk.org/jdk/pull/28891