[jdk8u] RFR: 8241311: Move some charset mapping tests from closed to open [v2]

[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.

Andrew John Hughes has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase.

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Changes:
  - all: https://git.openjdk.org/jdk8u/pull/44/files
  - new: https://git.openjdk.org/jdk8u/pull/44/files/c4850a96..c4850a96

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk8u&pr=44&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk8u&pr=44&range=00-01

  Stats: 0 lines in 0 files changed: 0 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