RFR: 8355632: WhiteBox.waitForReferenceProcessing() fails assert for return type [v2]

[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.)

Brent Christian has updated the pull request incrementally with four additional commits since the last revision:

 - move test
 - rename test class
 - enable assertions on test itself
 - use boolean.class

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/24892/files
  - new: https://git.openjdk.org/jdk/pull/24892/files/0931c0b3..b8dce1ae

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24892&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24892&range=00-01

  Stats: 61 lines in 3 files changed: 30 ins; 30 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