RFR: 8352969: G1: Improve testability of optional collections

[Guanqiang Han]

This PR introduces a new diagnostic flag EvacuateAllOptionalRegions to force G1 GC to evacuate all optional regions regardless of the predicted pause time. The motivation is to allow testing and validation of optional region evacuation behavior without being constrained by the remaining pause time.

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Commit messages:
 - add default value
 - Implement JDK-8352969

Changes: https://git.openjdk.org/jdk/pull/26880/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26880&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8352969
  Stats: 16 lines in 4 files changed: 3 ins; 2 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/26880.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26880/head:pull/26880

PR: https://git.openjdk.org/jdk/pull/26880