RFR: 8290043: serviceability/attach/ConcAttachTest.java failed "guarantee(!CheckJNICalls) failed: Attached JNI thread exited without being detached"

[David Holmes]

This bug was introduced by [JDK-8252921](https://bugs.openjdk.org/browse/JDK-8252921) which moved the unregister call from the thread destructor (because it was not called for all threads) to the post_run() method. But this totally overlooked native threads that attach and detach! This is easily fixed by moving the unregister call to the end of JavaThread::exit.

We also update the ProblemList and disable -Xcheck:JNI for a test that does actually terminate threads without detaching.

Testing: tiers 1-4

Thanks.

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Commit messages:
 - 8290043: serviceability/attach/ConcAttachTest.java failed "guarantee(!CheckJNICalls) failed: Attached JNI thread exited without being detached"

Changes: https://git.openjdk.org/jdk/pull/22924/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22924&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290043
  Stats: 12 lines in 3 files changed: 4 ins; 3 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/22924.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22924/head:pull/22924

PR: https://git.openjdk.org/jdk/pull/22924