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

[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

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

Commit messages:
 - fixed whitespace error
 - 8373643: Test serviceability/jvmti/vthread/ThreadListStackTracesTest/ThreadListStackTracesTest.java still failing

Changes: https://git.openjdk.org/jdk/pull/29102/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29102&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373643
  Stats: 7 lines in 1 file changed: 2 ins; 0 del; 5 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