RFR: 8307968: serviceability/jvmti/vthread/StopThreadTest/StopThreadTest.java timed out

[Serguei Spitsyn]

This is newly integrated test times out because it has a race in in the Test #A.1 and #A.2.
The main root cause is a print statement which can case target virtual thread to unpark and unmount.
This causes that the `StopThreads` unexpectedly fails with the `JVMTI_ERROR_OPAQUE_FRAME` error code.
The target thread can be in some other unexpected states if JVMTI `StopThread`
is called before the target thread method `A()` reached the synchronized statement.

The fix is to replace the `ensureStarted()` with the `ensureAtPointA()`.
The fix also includes some simplifications related to clearing the target thread interrupt status.

Testing:
Hundreds of mach5 runs of `serviceability/jvmti/vthread` tests which include the fixed `StopThreadTest`.
TBD: To run mack5 tiers1-3.

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Commit messages:
 - 8307968: serviceability/jvmti/vthread/StopThreadTest/StopThreadTest.java timed out

Changes: https://git.openjdk.org/jdk/pull/13969/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13969&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307968
  Stats: 20 lines in 1 file changed: 3 ins; 10 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/13969.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13969/head:pull/13969

PR: https://git.openjdk.org/jdk/pull/13969