RFR: Update periodic gc test for changed heuristic log message

[earthling-amzn]

This also includes a change to hold down regulator requests when the GC is already running in non-generational modes.

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Commit messages:
 - Hold down regulator requests when GC is already running
 - Update test for heuristic trigger message that includes generation name

Changes: https://git.openjdk.java.net/shenandoah/pull/21/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=21&range=00
  Stats: 12 lines in 2 files changed: 4 ins; 0 del; 8 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/21.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/21/head:pull/21

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