[jdk21u-dev] RFR: 8315889: Open source several Swing HTMLDocument related tests

[Jonathan Dowland]

This is a backport of [8f46abc9](https://github.com/openjdk/jdk/commit/8f46abc938ffe338e25d5fdbdcfa0aaa12edfa58) from [openjdk/jdk](https://git.openjdk.org/jdk). It introduces four newly open-sourced Swing tests and makes no other changes.

The tests all pass in jdk21u-dev.

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Commit messages:
 - Backport 8f46abc938ffe338e25d5fdbdcfa0aaa12edfa58

Changes: https://git.openjdk.org/jdk21u-dev/pull/529/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=529&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315889
  Stats: 250 lines in 4 files changed: 250 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/529.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/529/head:pull/529

PR: https://git.openjdk.org/jdk21u-dev/pull/529