[jdk21u-dev] RFR: 8369078: Fix faulty test conversion in IllegalCharsetName.java

[Goetz Lindenmaier]

I backport this as follow up of [JDK-8310049](https://bugs.openjdk.org/browse/JDK-8310049).

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Commit messages:
 - Backport 0f406c420e35f7a4358dc99711fd23d162f21777

Changes: https://git.openjdk.org/jdk21u-dev/pull/2301/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=2301&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/jdk21u-dev/pull/2301.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/2301/head:pull/2301

PR: https://git.openjdk.org/jdk21u-dev/pull/2301