RFR: 8334215: serviceability/dcmd/thread/PrintMountedVirtualThread.java failing with JTREG_TEST_THREAD_FACTORY=Virtual

[Inigo Mediavilla Saiz]

Follow up to https://github.com/openjdk/jdk/pull/19482 that was causing issues when the PrintMountedVirtualTest.java was
running with `JTREG_TEST_THREAD_FACTORY=Virtual` in the loom repo. 

- Fixes issues where the test observes the thread during transitions.
- Fixes a potential issue in the test where CountDownLatch.countDown unparks the main (virtual) thread and the main thread observes the dummy thread is transition .

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Commit messages:
 - 8334215: Fix PrintMountVThread if FACTORY=Virtual

Changes: https://git.openjdk.org/jdk/pull/19744/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19744&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334215
  Stats: 15 lines in 2 files changed: 4 ins; 3 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/19744.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19744/head:pull/19744

PR: https://git.openjdk.org/jdk/pull/19744