RFR: 8348865: JButton/bug4796987.java never runs because Windows XP is unavailable

[Alexey Ivanov]

The `javax/swing/JButton/4796987/bug4796987.java` test strictly verifies if it's run on Windows XP, for this reason it never runs on modern versions of Windows.

Since Windows XP is obsolete and visual styles cannot be disabled since Windows 8, I removed the OS version check completely.

I captured an image of the panel and analyse colors in the image instead of using `Robot.getPixelColor`; I save the image for reference if the test fails.

The updated test passes in the CI.

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Commit messages:
 - 8348865: JButton/bug4796987.java never runs because Windows XP is unavailable

Changes: https://git.openjdk.org/jdk/pull/23336/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23336&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8348865
  Stats: 78 lines in 1 file changed: 45 ins; 13 del; 20 mod
  Patch: https://git.openjdk.org/jdk/pull/23336.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23336/head:pull/23336

PR: https://git.openjdk.org/jdk/pull/23336