[jdk21u] RFR: 8316958: Add test for unstructured locking

[Aleksey Shipilev]

Clean backport to improve testing.

Additional testing:
 - [x] New test passes with different `LockingMode`-s

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Commit messages:
 - Backport bcafec54a52e4c0d92b075de461fcf16d6c100b4

Changes: https://git.openjdk.org/jdk21u/pull/263/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=263&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316958
  Stats: 60 lines in 1 file changed: 60 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u/pull/263.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/263/head:pull/263

PR: https://git.openjdk.org/jdk21u/pull/263