RFR: 8317228: GC: Make TestXXXHeapSizeFlags use createTestJvm

[Leo Korinth]

Minor testing done, I will test more later with other fixes.

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Commit messages:
 - 8317228: GC: Make TestXXXHeapSizeFlags use createTestJvm

Changes: https://git.openjdk.org/jdk/pull/15986/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15986&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317228
  Stats: 50 lines in 5 files changed: 35 ins; 5 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/15986.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15986/head:pull/15986

PR: https://git.openjdk.org/jdk/pull/15986