[jdk25u] RFR: 8369078: Fix faulty test conversion in IllegalCharsetName.java

[Justin Lu]

A test-only correction, which is a backport of commit [0f406c42](https://github.com/openjdk/jdk/commit/0f406c420e35f7a4358dc99711fd23d162f21777) from the [openjdk/jdk](https://git.openjdk.org/jdk) repository.

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Commit messages:
 - Backport 0f406c420e35f7a4358dc99711fd23d162f21777

Changes: https://git.openjdk.org/jdk25u/pull/293/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk25u&pr=293&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/jdk25u/pull/293.diff
  Fetch: git fetch https://git.openjdk.org/jdk25u.git pull/293/head:pull/293

PR: https://git.openjdk.org/jdk25u/pull/293