RFR: JDK-8317631: Refactor ChoiceFormat tests to use JUnit

[Justin Lu]

Please review this PR which refactors the ChoiceFormat tests to use JUnit.

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Commit messages:
 - init

Changes: https://git.openjdk.org/jdk/pull/16065/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16065&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317631
  Stats: 141 lines in 3 files changed: 73 ins; 39 del; 29 mod
  Patch: https://git.openjdk.org/jdk/pull/16065.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16065/head:pull/16065

PR: https://git.openjdk.org/jdk/pull/16065