RFR: 8306593: Fix nsk/jdi/stop/stop001 for virtual threads and remove from problem list

[Chris Plummer]

nsk/jdi/stop/stop001 is problem listed due to [JDK-7034630](https://bugs.openjdk.org/browse/JDK-7034630), but that issue doesn't seem to reproduce anymore. It's suspected that it was only an issue on Solaris. For this reason stop001 should be removed from the problem list. However, it will fail due to lacking virtual thread support, so this issue also needs to be fixed at the same time. The issue is that for virtual threads, after work for [JDK-8306434](https://bugs.openjdk.org/browse/JDK-8306434) and [JDK-8306471](https://bugs.openjdk.org/browse/JDK-8306471) is complete, the test needs to expect IllegalThreadStateException rather than the stop() call succeeding.

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Commit messages:
 - Added a comment.
 - Remove from problem list and fix virtual thread issues.

Changes: https://git.openjdk.org/jdk/pull/13966/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13966&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306593
  Stats: 52 lines in 3 files changed: 31 ins; 3 del; 18 mod
  Patch: https://git.openjdk.org/jdk/pull/13966.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13966/head:pull/13966

PR: https://git.openjdk.org/jdk/pull/13966