RFR: 8291429: java/lang/Thread/virtual/ThreadAPI.java timed out on single core system

[Alan Bateman]

This is a test only change for two tests for virtual threads that hang/timeout on single core systems. The two tests involve pinning and require at least two carrier threads. The test lib used by these tests is updated to define a new method that ensures parallelism is at least a given value and both tests are updated to use this. There are a number of tests in the debugger area that may potentially use this in the future.

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Commit messages:
 - Make close idempotent
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/10562/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10562&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291429
  Stats: 53 lines in 3 files changed: 47 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/10562.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10562/head:pull/10562

PR: https://git.openjdk.org/jdk/pull/10562