RFR: 8330862: GCBarrierIRExample fails when a different GC is selected via the command line

Tue Apr 23 06:38:47 UTC 2024

The example IR framework test introduced by [JDK-8330153](https://bugs.openjdk.org/browse/JDK-8330153) runs on (generational) ZGC only (`TestFramework.runWithFlags("-XX:+UseZGC", "-XX:+ZGenerational")`), but allows the jtreg user to select a conflicting GC externally, which causes the VM to fail. This changeset restricts the test so that it only runs if either ZGC or no GC is selected externally.

**Testing:** tier1-3, ongoing (windows-x64, linux-x64, linux-aarch64, and macosx-x64)

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Commit messages:
 - Enable test only when using generational ZGC

Changes: https://git.openjdk.org/jdk/pull/18906/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18906&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8330862
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/18906.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18906/head:pull/18906

PR: https://git.openjdk.org/jdk/pull/18906