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

[William Kemper]

The Shenandoah tier1 tests already run as part of the gc tier1 tests, so running all Shenandoah tiers repeats them. Running tier2 and tier3 tests separately should have them run in parallel and decrease overall time for GHA to complete.

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Commit messages:
 - Run shenandoah tier2 and tier3 tests in parallel

Changes: https://git.openjdk.org/shenandoah/pull/375/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=375&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/pull/375.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/375/head:pull/375

PR: https://git.openjdk.org/shenandoah/pull/375