[jdk21u-dev] RFR: 8310049: Refactor Charset tests to use JUnit

[Goetz Lindenmaier]

I backport this for parity with 21.0.10-oracle

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Commit messages:
 - Backport 09174e0c994dfb19fd09f551720c13c6479812d4

Changes: https://git.openjdk.org/jdk21u-dev/pull/2296/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=2296&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310049
  Stats: 2072 lines in 8 files changed: 425 ins; 376 del; 1271 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/2296.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/2296/head:pull/2296

PR: https://git.openjdk.org/jdk21u-dev/pull/2296