RFR: 8280913: Create a regression test for JRootPane.setDefaultButton() method

[Manukumar V S]

This tests the behaviour of the method JRootPane.setDefaultButton() in different platforms with different LAFs.
As per the spec https://docs.oracle.com/en/java/javase/17/docs/api/java.desktop/javax/swing/JRootPane.html#setDefaultButton(javax.swing.JButton), "The default button is the button which will be activated when a UI-defined activation event (typically the Enter key) occurs in the root pane regardless of whether or not the button has keyboard focus (unless there is another component within the root pane which consumes the activation event, such as a JTextPane). ".
This is currently not working in Linux platforms, may be this test could be problem-listed initially for linux-all.
This also fixes and colocates the tonga test Swing_JButton/Automated/bug4659800 written for verifying JDK-4659800.

-------------

Commit messages:
 - 8280913: Create a regression test for JRootPane.setDefaultButton() method

Changes: https://git.openjdk.java.net/jdk/pull/7278/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7278&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280913
  Stats: 123 lines in 2 files changed: 123 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7278.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7278/head:pull/7278

PR: https://git.openjdk.java.net/jdk/pull/7278