RFR: 8337847: [GenShen] jdk.test.whitebox.Whitebox.isObjectInOldGen misreports

[William Kemper]

Clean backport. Fix in test code.

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Commit messages:
 - Backport 44f5995bf87cb2e82176f2393517e4a11447b516

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/79/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=79&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337847
  Stats: 11 lines in 2 files changed: 9 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/79.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/79/head:pull/79

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/79