RFR: 8339960: GenShen: Fix inconsistencies in generational Shenandoah behavior [v3]

[Kelvin Nilsen]

> This pull request contains a backport of commit 1a7d597d from the openjdk/shenandoah repository.
> 
> The commit being backported was authored by Kelvin Nilsen on 13 Sep 2024 and was reviewed by William Kemper.

Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:

  Problem list two tests

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Changes:
  - all: https://git.openjdk.org/shenandoah-jdk21u/pull/90/files
  - new: https://git.openjdk.org/shenandoah-jdk21u/pull/90/files/e0a82e57..7bf48357

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=90&range=02
 - incr: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=90&range=01-02

  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/90.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/90/head:pull/90

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/90