RFR: 8373945: vmTestbase eatMemory/ClassUnloader provoke OOME to force GC and might cause GC in other threads [v12]

[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

SendaoYan has updated the pull request incrementally with one additional commit since the last revision:

  Remove unnecesary System.gc() and add sleep 1s to wait unload event happen

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/28891/files
  - new: https://git.openjdk.org/jdk/pull/28891/files/6c144309..6fc4bb95

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28891&range=11
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28891&range=10-11

  Stats: 9 lines in 1 file changed: 1 ins; 1 del; 7 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