RFR: 8317343: GC: Make TestHeapFreeRatio use createTestJvm

[Leo Korinth]

This fix is implicitly dependent on https://github.com/openjdk/jdk/pull/15986/files for `@requires opt.x` support. Initial testing passes, but I will do more (tier) testing before pushing.

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Commit messages:
 - 8317343: GC: Make TestHeapFreeRatio use createTestJvm

Changes: https://git.openjdk.org/jdk/pull/16007/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16007&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317343
  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/16007.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16007/head:pull/16007

PR: https://git.openjdk.org/jdk/pull/16007