RFR: 8310683: Refactor StandardCharset/standard.java to use JUnit [v6]

[Justin Lu]

> Please review this PR which is apart of the migration of JDK tests to JUnit. Standard.java is refactored to use JUnit, and thus split up into the following tests.
> 
> - typeTest
> - nameMethodTest
> - forNameMethodTest
> - charsetModifiersTest
> - correctCharsetsTest
> 
>  Additionally all unused methods were removed.

Justin Lu has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 11 commits:

 - Merge master
 - Just use charsetFields()
 - Review: reuse the standard charset fields
 - Merge branch 'pr/14599' of https://git.openjdk.org/jdk into JUnit-StandardCharsets
 - Merge branch 'master' into JDK-8310047-UTF-32
 - Merge pr/14599
 - Update test/jdk/java/nio/charset/StandardCharsets/Standard.java
   
   Co-authored-by: ExE Boss <3889017+ExE-Boss at users.noreply.github.com>
 - 8310047: Add UTF-32 based Charsets into StandardCharsets
 - Additional cleanup
 - Refactor Standard.java to use JUnit + other cleanup
 - ... and 1 more: https://git.openjdk.org/jdk/compare/401c3dea...71ab598a

-------------

Changes: https://git.openjdk.org/jdk/pull/14621/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14621&range=05
  Stats: 129 lines in 1 file changed: 42 ins; 28 del; 59 mod
  Patch: https://git.openjdk.org/jdk/pull/14621.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14621/head:pull/14621

PR: https://git.openjdk.org/jdk/pull/14621