[jdk17u-dev] RFR: 8310049: Refactor Charset tests to use JUnit

[Goetz Lindenmaier]

I backport this for parity with 17.0.18-oracle.

I had to resolve as [8260265: UTF-8 by Default](https://bugs.openjdk.org/browse/JDK-8260265) is not in 17.

The most notable difference is that I omitted the method defaultCharsetTest()
rather at the beginning, which replaces the try/catch added by 8260265 at the end.

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Commit messages:
 - backport 09174e0c994dfb19fd09f551720c13c6479812d4

Changes: https://git.openjdk.org/jdk17u-dev/pull/4015/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=4015&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310049
  Stats: 2047 lines in 8 files changed: 405 ins; 358 del; 1284 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/4015.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/4015/head:pull/4015

PR: https://git.openjdk.org/jdk17u-dev/pull/4015