[jdk21u-dev] RFR: 8326389: [test] improve assertEquals failure output

[Goetz Lindenmaier]

Let's backport this to fix the annoying message.

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Commit messages:
 - Backport 9b1f1e5294e130ec444b08af1f73d08e86fd91ee

Changes: https://git.openjdk.org/jdk21u-dev/pull/1787/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1787&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326389
  Stats: 4 lines in 1 file changed: 0 ins; 3 del; 1 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1787.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1787/head:pull/1787

PR: https://git.openjdk.org/jdk21u-dev/pull/1787