RFR: 8289740: Add verification testing during all concurrent phases in G1

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that adds a test that causes GCs that do lots of verification in all phases of the concurrent cycle to make sure that verification is working after changes.

Testing: test case, gha

Thanks,
  Thomas

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Depends on: https://git.openjdk.org/jdk/pull/9376

Commit messages:
 - Add test
 - Add test

Changes: https://git.openjdk.org/jdk/pull/9377/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9377&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289740
  Stats: 100 lines in 1 file changed: 100 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9377.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9377/head:pull/9377

PR: https://git.openjdk.org/jdk/pull/9377