RFR: 8300526: Test runtime/jni/IsVirtualThread/IsVirtualThread.java should exercise alternative virtual thread implementation

[Alan Bateman]

test/hotspot/jtreg/runtime/jni/IsVirtualThread/IsVirtualThread.java is updated to exercise JNI IsVirtualThread when running with -XX:-VMContinuations.  On ports with VMContinuations supported, the test re-runs with the alternative implementation of virtual threads to ensure that the JNI function works as expected.

While in the area, the test is changed to used the jtreg `@enablePreview` tag and also changed to put a try-finally around the test for a started thread to ensure it doesn't it unparks in the event that the test fails.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/12070/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12070&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300526
  Stats: 26 lines in 1 file changed: 16 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/12070.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12070/head:pull/12070

PR: https://git.openjdk.org/jdk/pull/12070