RFR: 8302899: Executors.newSingleThreadExecutor can use Cleaner to shutdown executor

[Alan Bateman]

Executors.newSingleThreadExecutor returns a delegating ExecutorService that has finalizer to shutdown the underlying TPE when the wrapper is finalizable. It goes back to JDK 6 and JDK-6399443. This is the last non-empty finalizer in java.base. Removing it will likely lead to bug reports/complaints as the current behavior goes back to 2006. So the proposal is to just replace it with a Cleaner, trivially done in this case. As part of the changes, I've replaced the existing test with a more modern test that exercises more scenarios.

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Commit messages:
 - Replace older test
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/12675/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12675&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302899
  Stats: 147 lines in 2 files changed: 52 ins; 19 del; 76 mod
  Patch: https://git.openjdk.org/jdk/pull/12675.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12675/head:pull/12675

PR: https://git.openjdk.org/jdk/pull/12675