[jdk21u-dev] RFR: 8320707: Virtual thread test updates

[Aleksey Shipilev]

Test-only backport that simplifies Loom maintenance. The original commit applies cleanly, but it does not work out of the box, because `VThreadPinner` uses FFM, which is a preview feature in JDK 21, made final in JDK 22. I believe it is safe to rely on FFM for these tests, and the only thing we need to do is to add `@enablePreview` in relevant tests. See the additional commits on top that do it.

This would also make subsequent backports clean.

Additional testing:
 - [x] MacOS AArch64 server fastdebug, `jdk_loom hotspot_loom`
 - [ ] Linux AArch64 server fastdebug, `all`
 - [ ] Linux x86_64 server fastdebug, `all`

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Commit messages:
 - Enable preview in affected tests
 - Backport b67b71cd87c62f15d5b73f923c300d0f77c988f5

Changes: https://git.openjdk.org/jdk21u-dev/pull/328/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=328&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320707
  Stats: 989 lines in 16 files changed: 626 ins; 213 del; 150 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/328.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/328/head:pull/328

PR: https://git.openjdk.org/jdk21u-dev/pull/328