RFR: 8346051: MemoryTest fails when Shenandoah's generational mode is enabled

[William Kemper]

Clean backport. Test only fix.

-------------

Commit messages:
 - Backport cfa04d31ddff49cbf5dfdfedd61264d5556a283c

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/146/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=146&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8346051
  Stats: 16 lines in 1 file changed: 12 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/146.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/146/head:pull/146

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/146