RFR: 8358158: test/jdk/java/io/Console/CharsetTest.java failing with NoClassDefFoundError: jtreg/SkippedException

[Naoto Sato]

Fixing a regression caused by the fix to JDK-8356985. Although the fix in `CharsetTest` was a clean-up and not the gist of the original issue, the change seem to have caused not finding `SkippedException` at runtime in certain cases. Changing the test to JUnit based so that the offending `SkippedException` can be replaced with JUnit's `Assumptions`. Also renamed the test case itself to reflect what's actually tested in it.

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

Commit messages:
 - args -> classpath
 - Renamed the test
 - initial commit

Changes: https://git.openjdk.org/jdk/pull/25601/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25601&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358158
  Stats: 217 lines in 4 files changed: 112 ins; 105 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/25601.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25601/head:pull/25601

PR: https://git.openjdk.org/jdk/pull/25601