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

[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.

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Commit messages:
 - Additional cleanup
 - Refactor Standard.java to use JUnit + other cleanup
 - Fix imports and update copyright year

Changes: https://git.openjdk.org/jdk/pull/14621/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14621&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310683
  Stats: 118 lines in 1 file changed: 40 ins; 23 del; 55 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