RFR: 8376051: gc/stress/TestStressG1Uncommit.java fails assertLessThan: expected that xxx < xxx

[SendaoYan]

Hi all,

[JDK-8238686](https://bugs.openjdk.org/browse/JDK-8238686) change the default value MinHeapFreeRatio/MaxHeapFreeRatio to 0/100, this change make G1 never uncommit heap memory to OS system, this will make test test/hotspot/jtreg/gc/stress/TestStressG1Uncommit.java fails "assertLessThan: expected that xxx < xxx", and the tested java process never print "Uncommit regions" which checked by this test.

This PR set MinHeapFreeRatio/MaxHeapFreeRatio to the original value 40/70 to make this test work expectly, and Use WhileBox.fullGC() instead of System.gc() which will make test more robustness.

Change has been verified locally on linux-x64 by run this tests both with release build and fastdebug build.

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Commit messages:
 - 8376051: gc/stress/TestStressG1Uncommit.java fails assertLessThan: expected that xxx < xxx

Changes: https://git.openjdk.org/jdk/pull/29357/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29357&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8376051
  Stats: 14 lines in 1 file changed: 11 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/29357.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29357/head:pull/29357

PR: https://git.openjdk.org/jdk/pull/29357