<i18n dev> RFR: 8310631: test/jdk/sun/nio/cs/TestCharsetMapping.java is spuriously passing

[Naoto Sato]

Fixed the failing (well, false-positive) test case. Made the following changes to the test:

- Corrected the path to the mapping files directory
- Made sure to fail if the directory path is incorrect
- Took care of `GB18030` alias, which is dynamically derived at runtime
- Provided `MS950_HKSCS.map`, which is simply a copy of `HKSCS2008.map`
- Excluded other failing tests for IBM charsets that do not have map files.

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Commit messages:
 - initial commit

Changes: https://git.openjdk.org/jdk/pull/15807/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15807&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310631
  Stats: 20 lines in 2 files changed: 14 ins; 2 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/15807.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15807/head:pull/15807

PR: https://git.openjdk.org/jdk/pull/15807