RFR: 8349689: Several virtual thread tests missing /native keyword

[SendaoYan]

H all,

This PR add `/native` keyword in the test header for virtual thread tests. The `/native` keyword will make run the related tests by jtreg standalone more friendly.

I runed all the tests without -nativepath argument and find the fail tests. This will find all the virtual thread tests which missing '/native' flag.

1. java/lang/management/ThreadMXBean/VirtualThreads.java#default

VirtualThreads.java:223 call "invoker()" in test/lib/jdk/test/lib/thread/VThreadPinner.java file, which will call the native function "call".
java/lang/management/ThreadMXBean/VirtualThreads.java#no-vmcontinuations run this test with VM option "-XX:-VMContinuations" and `assumeTrue(VThreadScheduler.supportsCustomScheduler(), "No support for custom schedulers");` will make junit skip the 'testGetThreadInfoCarrierThread' unit test, so 'VirtualThreads.java#no-vmcontinuations' do not need '/native' keywork.

2. java/lang/Thread/virtual/stress/PinALot.java

PinALot.java:58 call "invoker()" in test/lib/jdk/test/lib/thread/VThreadPinner.java file, which will call the native function "call".

3. java/lang/Thread/virtual/SynchronizedNative.java

Call System.loadLibrary("SynchronizedNative") at line 85.

4. Tests java/lang/Thread/virtual/stress/GetStackTraceALotWhenPinned.java, java/lang/Thread/virtual/ThreadAPI.java, java/lang/Thread/virtual/RetryMonitorEnterWhenPinned.java, java/lang/Thread/virtual/MonitorWaitNotify.java, java/lang/Thread/virtual/MonitorPinnedEvents.java, java/lang/Thread/virtual/MonitorEnterExit.java and other tests are similar to java/lang/Thread/virtual/stress/PinALot.java


- Why we need the '/native' keywork?

I usually run the tests through jtreg directively, rather than run the tests through make test. If I run the test which load the native shared library files and the test missing '/native' keyword but I forgot to pass the '-nativepath' argument to jtreg, or pass the incorrect '-nativepath' argument to jtreg, sometimes it will report timeouted after a long time, such as java/lang/Thread/virtual/JfrEvents.java. If we add the missing '/native' keywork, jtreg will report 'Error. Use -nativepath to specify the location of native code' right away. So add the missing '/native' keyword will make run the tests more friendly.

Change has been verified locally, test-fix, no risk.

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Commit messages:
 - 8349689: Several virtual thread tests missing /native keyword

Changes: https://git.openjdk.org/jdk/pull/23550/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23550&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349689
  Stats: 48 lines in 10 files changed: 0 ins; 0 del; 48 mod
  Patch: https://git.openjdk.org/jdk/pull/23550.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23550/head:pull/23550

PR: https://git.openjdk.org/jdk/pull/23550