RFR: 8335932: GenShen: Fix old heuristic unit test

[William Kemper]

Clean backport, test only, low risk.

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Commit messages:
 - Backport f0b877138a7dd45fc957178de23cffe6e32405a0

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/64/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=64&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335932
  Stats: 12 lines in 1 file changed: 6 ins; 3 del; 3 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/64.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/64/head:pull/64

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/64