RFR: 8355632: WhiteBox.waitForReferenceProcessing() fails assert for return type

[Brent Christian]

The newly-added `WhiteBox.waitForReferenceProcessing()` (see [8305186](https://bugs.openjdk.org/browse/JDK-8305186)) always fails with assertions enabled.
I've updated the assertion, and also added the test I used locally to test the new method (just not with assertions enabled, apparently.)

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Commit messages:
 - fix assert, add test case

Changes: https://git.openjdk.org/jdk/pull/24892/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24892&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8355632
  Stats: 31 lines in 2 files changed: 30 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/24892.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24892/head:pull/24892

PR: https://git.openjdk.org/jdk/pull/24892