RFR: 8301570: Test runtime/jni/nativeStack/ needs to detach the native thread

[David Holmes]

Please review this simple fix to a test to detach the native thread that we created and attached. Failing to do so can cause problems on some platforms.

Thanks.

Testing: itself, tiers 1-3 sanity

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Commit messages:
 - 8301570: Test runtime/jni/nativeStack/ needs to detach the native thread

Changes: https://git.openjdk.org/jdk/pull/12357/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12357&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301570
  Stats: 8 lines in 1 file changed: 7 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12357.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12357/head:pull/12357

PR: https://git.openjdk.org/jdk/pull/12357