RFR: 8301684: Fix test code to not get finalizer deprecation warnings [v3]

[Afshin Zafari]

> To replace the finalizer use in the code, the `Cleaner` approach is used as stated in [Oracle doc on deprecated finalize() method](https://docs.oracle.com/javase/10/docs/api/java/lang/Object.html#finalize()).
> Briefly:
> 1.  An instance of `Cleaner` (`java.lang.ref`) is created.
> 2. Using the `Cleaner`, an object is registered with a `Runnable` callback that is notified when the object is no longer reachable (GC'ed).
> 3. Write code in the callback to do other cleanings.
> 
> 
> Cleaner c = new Cleaner();
> Cleanable cleanable = c.register(an_obj, a_runnable);
> ...
>   //JVM notifies by calling the 
>     a_runnable.run() {...}
> 
> ### Tests
> local: 
>      vmTestbase/nsk/monitoring/stress/classload
>       vmTestbase/nsk/jvmti/AttachOnDemand
>       vmTestbase/nsk/jdi/ObjectReference/referringObjects
>       vmTestbase/nsk/sysdict
> mach5: tier 1-5

Afshin Zafari has updated the pull request incrementally with one additional commit since the last revision:

  8301684: Fix test code to not get finalizer deprecation warnings

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/12968/files
  - new: https://git.openjdk.org/jdk/pull/12968/files/15c2ef56..58d24335

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12968&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12968&range=01-02

  Stats: 12 lines in 1 file changed: 0 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/12968.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12968/head:pull/12968

PR: https://git.openjdk.org/jdk/pull/12968