RFR: 8289739: Add G1 specific GC breakpoints for testing

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that adds a few G1 specific GC breakpoints for future testing [JDK-8289740](https://bugs.openjdk.org/browse/JDK-8289740).

Testing: gha, local testing

Thanks,
  Thomas

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Commit messages:
 - Initial change, adding new concurrent cycle breakpoints

Changes: https://git.openjdk.org/jdk/pull/9376/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9376&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289739
  Stats: 23 lines in 3 files changed: 23 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9376.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9376/head:pull/9376

PR: https://git.openjdk.org/jdk/pull/9376