[jdk21u-dev] RFR: 8316149: Open source several Swing JTree JViewport KeyboardManager tests

[Goetz Lindenmaier]

I combine this with follow-up 8316627 JViewport Test headless failure.

This are two clean backports.

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Commit messages:
 - Backport 8316627 JViewport Test headless failure
 - Backport a021dbcb9e58ab60abfc8c46ff25f3aa8ce3cc9f

Changes: https://git.openjdk.org/jdk21u-dev/pull/1255/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1255&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316149
  Stats: 370 lines in 5 files changed: 370 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1255.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1255/head:pull/1255

PR: https://git.openjdk.org/jdk21u-dev/pull/1255