RFR: 8352969: G1: Improve testability of optional collections [v3]

[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.

Guanqiang Han has updated the pull request incrementally with one additional commit since the last revision:

  Update g1YoungCollector.cpp
  
  format fix

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Changes:
  - all: https://git.openjdk.org/jdk/pull/26880/files
  - new: https://git.openjdk.org/jdk/pull/26880/files/6661deef..fc13c5e0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26880&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26880&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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