RFR: 8167252: Some of Charset.availableCharsets() does not contain itself [v2]

[Iris Clark]

On Wed, 14 Jun 2023 18:08:34 GMT, Naoto Sato <naoto at openjdk.org> wrote:

>> Adding themselves into their `contains()` method will fix it.
>
> Naoto Sato has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Refined the test

Marked as reviewed by iris (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/14473#pullrequestreview-1480141268