RFR: 8373253: Re-work InjectGCWorkerCreationFailure for future changes

[Leo Korinth]

This PR slightly changes when we may `InjectGCWorkerCreationFailure`.  At the moment we wait until `is_init_completed()`. I am changing this to instead be after `_created_workers > 0`. The reason is that we might in the future, https://bugs.openjdk.org/browse/JDK-8367993,  create even more threads "on demand", and if so, we would fail VM creation if we inject worker creation failures after the VM is initiated but before we have created any of the worker threads.

This change should not change the current behaviour. But help future improvements.

I have tested this on `test/hotspot/jtreg/gc/stress/gcold/TestGCOldWithG1.java`, the only place where this flag is tested to my knowledge.

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Commit messages:
 - 8373253: Re-work InjectGCWorkerCreationFailure for future changes

Changes: https://git.openjdk.org/jdk/pull/28719/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28719&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373253
  Stats: 10 lines in 1 file changed: 7 ins; 3 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/28719.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28719/head:pull/28719

PR: https://git.openjdk.org/jdk/pull/28719