RFR: 8310049: Refactor Charset tests to use JUnit [v5]

[Justin Lu]

> As discussed in https://github.com/openjdk/jdk/pull/14473/files, tests within _test/jdk/java/nio/charset/Charset_ could benefit from using a test framework such as JUnit.

Justin Lu has updated the pull request incrementally with one additional commit since the last revision:

  Review: Make null charset its own test

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Changes:
  - all: https://git.openjdk.org/jdk/pull/14500/files
  - new: https://git.openjdk.org/jdk/pull/14500/files/6a39ceab..bc50c6f5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14500&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14500&range=03-04

  Stats: 52 lines in 2 files changed: 44 ins; 7 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14500.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14500/head:pull/14500

PR: https://git.openjdk.org/jdk/pull/14500