RFR: 8334085: Test crash: assert(thread->held_monitor_count() == 0) failed: Must be

[Serguei Spitsyn]

The test `serviceability/jvmti/GetOwnedMonitorInfo/GetOwnedMonitorInfoTest.java` is failing with the assert in the `thaw()` function. The assert is not fully correct as it does not account for an unexpected scenario.

Thanks to Patricio for reproducing this failure and identifying the root cause:
> The problem is that we can unmount a virtual thread, then mount it again, thaw a few frames, execute code that acquires a JNI monitor, and then call thaw again without releasing that monitor. In this test this will happen if the vthread is unmounted in System.out.println("Thread doing JNI call: " ...) because of contention with the main thread doing System.out.println("Main waiting for event.").
The issue can be reproduced by adding Thread.yield() before jniMonitorEnterAndLetObjectDie(). 

The fix corrects the assert to account for the `thread->jni_monitor_count()`.
Also, the fix includes the test tweak described above which makes this failure always reproducible.

Testing:
 - Ran the test `GetOwnedMonitorInfoTest.java` locally
 - Mach5 tiers 1-6 are passed

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Commit messages:
 - 8334085: Test crash: assert(thread->held_monitor_count() == 0) failed: Must be

Changes: https://git.openjdk.org/jdk/pull/20294/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20294&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334085
  Stats: 3 lines in 2 files changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/20294.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20294/head:pull/20294

PR: https://git.openjdk.org/jdk/pull/20294