[jdk21u-dev] RFR: 8317631: Refactor ChoiceFormat tests to use JUnit

[Goetz Lindenmaier]

I backport this for parity with 21.0.4-oracle.

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Commit messages:
 - Backport 5e8d893c4809fdb30e01bdfe06b4f488f7bbef83

Changes: https://git.openjdk.org/jdk21u-dev/pull/382/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=382&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317631
  Stats: 165 lines in 3 files changed: 90 ins; 38 del; 37 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/382.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/382/head:pull/382

PR: https://git.openjdk.org/jdk21u-dev/pull/382