RFR: 8310049: Refactor Charset tests to use JUnit

[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.

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Commit messages:
 - Merge master and fix conflicts in Contains.java
 - Refactor CharsetContainmentTest.java
 - Remove unusued JUnit import in RegisteredCharsets.java
 - Refactor contains.java
 - Move bug id for default charset test
 - Add bug id to EncDec.java
 - Do not use wildcard import in IllegalCharsetName.java
 - Refactor RegisteredCharsets.java
 - Move default charset test to IllegalCharsetName.java
 - Refactor EncDec test to use JUnit
 - ... and 5 more: https://git.openjdk.org/jdk/compare/3e0bbd29...be352ca2

Changes: https://git.openjdk.org/jdk/pull/14500/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14500&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310049
  Stats: 2076 lines in 9 files changed: 366 ins; 447 del; 1263 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