RFR: 8291651: CleanerTest.java fails with "Cleanable was cleaned" [v2]

[Roger Riggs]

> CleanerTest is failing intermittently on Aarch64, with -Xcomp, and using a VirtualThread for the Cleaner in the test.
> The extensively relies on references processing and invokes GC on a very short cycle.
> The existing 10ms delay between GC requests is too short for the changes to reference states to be reliably visible.
> 
> The delay between gc requests is extended to 200ms and the tests pass reliably.
> There are small improvements in diagnostic messages and cleanup of unused imports.

Roger Riggs has updated the pull request incrementally with one additional commit since the last revision:

  Cleanup of messages that describe the cleaning function to be run or not.
  Switched to using jtreg @enablePreview.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/10048/files
  - new: https://git.openjdk.org/jdk/pull/10048/files/8dee8e8f..d98a3b02

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10048&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10048&range=00-01

  Stats: 11 lines in 1 file changed: 1 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/10048.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10048/head:pull/10048

PR: https://git.openjdk.org/jdk/pull/10048