RFR: JDK-8315889: Open source several Swing HTMLDocument related tests

[Harshitha Onkar]

Following tests are being open sourced as part of this PR.

javax/swing/text/html/HTMLDocument/4251593/bug4251593.java
javax/swing/text/html/HTMLDocument/4687405/bug4687405.java
javax/swing/text/html/HTMLDocument/4226914/bug4226914.java
javax/swing/text/html/HTMLEditorKit/4213373/bug4213373.java

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Commit messages:
 - HTMLDoc tests

Changes: https://git.openjdk.org/jdk/pull/15687/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15687&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/jdk/pull/15687.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15687/head:pull/15687

PR: https://git.openjdk.org/jdk/pull/15687