[jdk21u-dev] RFR: 8316061: Open source several Swing RootPane and Slider related tests

[Goetz Lindenmaier]

I backport this for parity with 21.0.8-oracle

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Commit messages:
 - Backport 138542de7889e8002df0e15a79e31d824c6a0473

Changes: https://git.openjdk.org/jdk21u-dev/pull/1483/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1483&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316061
  Stats: 303 lines in 5 files changed: 303 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1483.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1483/head:pull/1483

PR: https://git.openjdk.org/jdk21u-dev/pull/1483