RFR: 8305081: Remove finalize() from test/hotspot/jtreg/compiler/runtime/Test8168712

[Afshin Zafari]

- The `finalize()` method is replaced with `cleanup()`. 
- A new constructor is added to register the cleanup method.
- A static `Cleaner` is defined to have only one cleaner thread for all the 15000 instances. Otherwise, we get an `OutOfMemoryException` on cleaner thread creation.

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Commit messages:
 - 8305081: Remove finalize() from test/hotspot/jtreg/compiler/runtime/Test8168712

Changes: https://git.openjdk.org/jdk/pull/13886/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13886&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305081
  Stats: 13 lines in 1 file changed: 11 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/13886.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13886/head:pull/13886

PR: https://git.openjdk.org/jdk/pull/13886