RFR: Update periodic gc test for changed heuristic log message

[Roman Kennke]

On Thu, 11 Mar 2021 23:53:58 GMT, earthling-amzn <github.com+71722661+earthling-amzn 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 :-)

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Marked as reviewed by rkennke (Reviewer).

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