[jdk21u-dev] RFR: 8317228: GC: Make TestXXXHeapSizeFlags use createTestJvm

[Goetz Lindenmaier]

I backport this to keep the 21u test suite up-to-date. This will simplify future test backports.

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Commit messages:
 - Backport 7ca0ae94159ac0fd2df23ee1a1e8cf626ce31048

Changes: https://git.openjdk.org/jdk21u-dev/pull/53/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=53&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317228
  Stats: 56 lines in 6 files changed: 40 ins; 6 del; 10 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/53.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/53/head:pull/53

PR: https://git.openjdk.org/jdk21u-dev/pull/53