RFR: 8373643: Test serviceability/jvmti/vthread/ThreadListStackTracesTest/ThreadListStackTracesTest.java still failing [v2]

[Serguei Spitsyn]

> The test is still failing after the fix of [JDK-8371502](https://bugs.openjdk.org/browse/JDK-8371502). I suspect the issue is in the `ReentrantLock` implementation but suggest to make one more update of this test to make it more clear.
> The test update includes the following changes:
>  - update method `ensureReadyAndWaiting()`:
>     - add `sleep(50)` at start of method
>     - replace call to `rlock.hasQueuedThreads()` with call to `rlock.hasQueuedThread(vt)`
>  - update method `checkStates()` to make it more stable and tracing output more clear
>  
>  Testing:
>   - TBD: mach5 tiers 1-3

Serguei Spitsyn has updated the pull request incrementally with one additional commit since the last revision:

  review: tweak one of the comments

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/29102/files
  - new: https://git.openjdk.org/jdk/pull/29102/files/3850f374..3ff0d66e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=29102&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=29102&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/29102.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29102/head:pull/29102

PR: https://git.openjdk.org/jdk/pull/29102