<i18n dev> RFR: 8347146: Convert IncludeLocalesPluginTest to use JUnit

[Naoto Sato]

A simple refinement to the test makes diagnosing failures easier.

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Commit messages:
 - initial commit

Changes: https://git.openjdk.org/jdk/pull/23016/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23016&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8347146
  Stats: 423 lines in 1 file changed: 22 ins; 40 del; 361 mod
  Patch: https://git.openjdk.org/jdk/pull/23016.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23016/head:pull/23016

PR: https://git.openjdk.org/jdk/pull/23016