RFR: Update periodic gc test for changed heuristic log message

[earthling-amzn]

On Fri, 12 Mar 2021 19:22:32 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> This also includes a change to hold down regulator requests when the GC is already running in non-generational modes.
>
> Looks ok.
> I was just thinking that GCMode kinda clashes with the ShenandoahGCMode (satb, iu, generational) distinction, and is thus a little confusing. It *did* confuse me for a bit :-)

What about `GCPhase` or `GCStage`?

Another enumeration that warrants some discussion: `GenerationMode` - which is confusingly similar to the class name `ShenandoahGenerationalMode` (and the class `ShenandoahGeneration`). I think at the least it should have the Shenandoah prefix, or perhaps declare it inside `ShenandoahGenerationalMode` as just `Generation`?

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PR: https://git.openjdk.java.net/shenandoah/pull/21