[jdk21u-dev] RFR: 8310631: test/jdk/sun/nio/cs/TestCharsetMapping.java is spuriously passing

[Goetz Lindenmaier]

I backport this for parity with 21.0.3-oracle.

-------------

Commit messages:
 - Backport e3201d1d13433857a1b34ff0ca93f9ae1a4e22aa

Changes: https://git.openjdk.org/jdk21u-dev/pull/169/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=169&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310631
  Stats: 20 lines in 2 files changed: 14 ins; 2 del; 4 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/169.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/169/head:pull/169

PR: https://git.openjdk.org/jdk21u-dev/pull/169