[jdk21u-dev] RFR: 8316104: Open source several Swing SplitPane and RadioButton related tests

[Andrew Lu]

I backport this for parity with 21.0.5-oracle.

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Commit messages:
 - Backport f52e500f806085f9645cb7857cc7b4e648685351

Changes: https://git.openjdk.org/jdk21u-dev/pull/661/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=661&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316104
  Stats: 332 lines in 3 files changed: 332 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/661.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/661/head:pull/661

PR: https://git.openjdk.org/jdk21u-dev/pull/661