RFR: 8310049: Refactor Charset tests to use JUnit [v4]

[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. 
> 
> In addition, this PR groups the emptyCharset, nullCharset, and defaultCharset tests into _illegalCharsets.java_. The _default.java_ file was removed, as it did not test anything.

Justin Lu has updated the pull request incrementally with three additional commits since the last revision:

 - Review: Move default test back to RegisteredCharsets.java
 - Review: rename standard charsets test
 - Revert "Remove Default test"
   
   This reverts commit 346cb4c050adb8bd8a3cb942e25a98484703b997.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/14500/files
  - new: https://git.openjdk.org/jdk/pull/14500/files/6c7a8d05..6a39ceab

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14500&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14500&range=02-03

  Stats: 61 lines in 4 files changed: 46 ins; 8 del; 7 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