RFR: 8334215: serviceability/dcmd/thread/PrintMountedVirtualThread.java failing with JTREG_TEST_THREAD_FACTORY=Virtual [v2]

[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 .

Inigo Mediavilla Saiz has updated the pull request incrementally with one additional commit since the last revision:

  Improve assert and document conditional print

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/19744/files
  - new: https://git.openjdk.org/jdk/pull/19744/files/63a8cb6b..9a954efe

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19744&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19744&range=00-01

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