RFR: JDK-8315876 Open source several Swing CSS related tests

[Harshitha Onkar]

Following tests are open sourced as part of this PR.

1. javax/swing/text/html/CSS/4174871/bug4174871.java
2. javax/swing/text/html/CSS/4174874/bug4174874.java
3. javax/swing/text/html/CSS/4284162/bug4284162.java
4. javax/swing/text/html/CSS/4764897/bug4764897.java
5. javax/swing/text/html/HTMLDocument/4209280/bug4209280.java

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Commit messages:
 - CSS files

Changes: https://git.openjdk.org/jdk/pull/15769/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15769&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315876
  Stats: 456 lines in 5 files changed: 456 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15769.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15769/head:pull/15769

PR: https://git.openjdk.org/jdk/pull/15769