RFR: 8310683: Refactor StandardCharset/standard.java to use JUnit [v5]

[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 incrementally with two additional commits since the last revision:

 - Just use charsetFields()
 - Review: reuse the standard charset fields

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Changes:
  - all: https://git.openjdk.org/jdk/pull/14621/files
  - new: https://git.openjdk.org/jdk/pull/14621/files/86c86cad..0ac05e25

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14621&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14621&range=03-04

  Stats: 6 lines in 1 file changed: 3 ins; 0 del; 3 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