RFR: 8322347: GenShen: Run shenandoah tier2 and tier3 tests separately in GHA

[William Kemper]

Clean backport

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Commit messages:
 - Backport b415cec7f1728f17a838a3711ac462b1717424d1

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/15/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=15&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322347
  Stats: 8 lines in 1 file changed: 5 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/15.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/15/head:pull/15

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/15