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

[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.

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

Commit messages:
 - Extended timeout between GC requests to adapt to different timing on Aarch64.
 - 8291651: CleanerTest.java fails with "Cleanable was cleaned"
 - 8291651: CleanerTest.java fails with "Cleanable was cleaned"

Changes: https://git.openjdk.org/jdk/pull/10048/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10048&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291651
  Stats: 9 lines in 1 file changed: 2 ins; 3 del; 4 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