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

[Tobias Hartmann]

On Tue, 23 Apr 2024 06:29:57 GMT, Roberto Castañeda Lozano <rcastanedalo at openjdk.org> wrote:

> 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)

Looks good and trivial.

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Marked as reviewed by thartmann (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/18906#pullrequestreview-2016349327