RFR: 8308429: jvmti/StopThread/stopthrd007 failed with "NoClassDefFoundError: Could not initialize class jdk.internal.misc.VirtualThreads"

[Leonid Mesnik]

The test fails because ThreadDeath is raised during class jdk.internal.misc.VirtualThreads initialization. The proposed fix is to pre-initialize this step to avoid such failures. See more details in the bug.
I reproduced the original problem and verified that it is not reproduced after fix. 
Tested with tier5 and running nsk/jvmti tests with and without virtual test thread factory.

I don't think that more complex fix is needed. There is a plan to review nsk/jvmti stopThread tests and see if 
./serviceability/jvmti/vthread/StopThreadTest/StopThreadTest.java 
might be improved to cover them.

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

Commit messages:
 - fix

Changes: https://git.openjdk.org/jdk/pull/15966/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15966&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308429
  Stats: 6 lines in 1 file changed: 5 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/15966.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15966/head:pull/15966

PR: https://git.openjdk.org/jdk/pull/15966