[jdk21u-dev] RFR: 8315951: Open source several Swing HTMLEditorKit related tests

[Goetz Lindenmaier]

I backport this for parity with 21.0.8-oracle

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Commit messages:
 - Backport d19e017d3fd87a6b7613f44a3757d574a6560680

Changes: https://git.openjdk.org/jdk21u-dev/pull/1558/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1558&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315951
  Stats: 365 lines in 4 files changed: 365 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1558.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1558/head:pull/1558

PR: https://git.openjdk.org/jdk21u-dev/pull/1558