[jdk8u] RFR: 8241311: Move some charset mapping tests from closed to open

[Andrew John Hughes]

This is a second pre-requisite for [JDK-8301119](https://bugs.openjdk.org/browse/JDK-8301119) ("Support for GB18030-2022") and so is being proposed for inclusion in 8u382 during rampdown, so that the changes are in place for when GB18030-2022 enforcement begins in August. It adds more tests for character mappings, including GB18030.

The patch only introduces new tests, shuffled to the correct location. The one change to an existing file (`TestCharsetMapping.java`) is dropped, as it only adds an unneeded `@modules` line. Similarly, `@modules` is dropped from `CoderTest.java`, `ConverterTest.java` and `TestConv.java`.

The remaining change is to `MS950.c2b-irreversible`. 8u does not have [JDK-8232161](https://bugs.openjdk.org/browse/JDK-8232161), a change with an associated CSR that changed some of the mappings in the MS950 character set. We removed the four mappings from the test set, so as to match 8u's behaviour and avoid the `CoderTest.java` test from failing.

All `sun.nio.cs` tests pass with this patch applied.

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Depends on: https://git.openjdk.org/jdk8u/pull/43

Commit messages:
 - Backport c809c9944400f7ee9bc296e40d6068273bde5912

Changes: https://git.openjdk.org/jdk8u/pull/44/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk8u&pr=44&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8241311
  Stats: 732192 lines in 166 files changed: 732192 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk8u/pull/44.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u.git pull/44/head:pull/44

PR: https://git.openjdk.org/jdk8u/pull/44