RFR: JDK-8282778: Create a regression test for JDK-4699544

[Srinivas Mandalika]

Create a regression test for JDK-4699544

The subclass of javax.swing.JRootPane (AccessibleJRootPane) that implements the accessibility interface javax.accessibility.AccessibleComponent is derived from java.awt.AccessibleAWTComponent, which returns null for getAccessibleAt() because a component does not necessarily have childs.

The root pane always has a content pane child, so getAccessibleAt() should be overwritten by AccessibleJRootPane appropriately.
The test added verifies the same.

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Commit messages:
 - 8282778: Create a regression test for JDK-4699544

Changes: https://git.openjdk.java.net/jdk/pull/7739/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7739&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282778
  Stats: 57 lines in 1 file changed: 57 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7739.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7739/head:pull/7739

PR: https://git.openjdk.java.net/jdk/pull/7739