[jdk21u-dev] RFR: 8354285: Open source Swing tests Batch 3

[Satyen Subramaniam]

Backporting JDK-8354285: Open source Swing tests Batch 3. Adds three UI tests (split pane, deliminators, checkbox menu items). Ran GHA Sanity Checks, local Tier 1 and 2, and new tests directly. Patch is clean. Backporting for parity with Oracle.

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Commit messages:
 - Backport 4c99489420bd73159eca6bae22442f7b29156c1d

Changes: https://git.openjdk.org/jdk21u-dev/pull/2125/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=2125&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8354285
  Stats: 253 lines in 3 files changed: 253 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/2125.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/2125/head:pull/2125

PR: https://git.openjdk.org/jdk21u-dev/pull/2125