RFR: 8369078: Fix faulty test conversion in IllegalCharsetName.java

[Justin Lu]

This PR corrects two instances of `Charset.isSupported` which were inadvertently converted to `Charset.forName` in _test/jdk/java/nio/charset/Charset/IllegalCharsetName.java_ during [JDK-8369078](https://bugs.openjdk.org/browse/JDK-8369078).

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Commit messages:
 - init

Changes: https://git.openjdk.org/jdk/pull/27631/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27631&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369078
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/27631.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27631/head:pull/27631

PR: https://git.openjdk.org/jdk/pull/27631