RFR: 8283507: Create a regression test for RFE 4287690

[Manukumar V S]

Create a regression test for [JDK-4287690](https://bugs.openjdk.java.net/browse/JDK-4287690)
Issue description:
I want JComboBox to send an event just before the popup (drop down) list is displayed.

Fix:
This requires some additional API targeted for major release of the JDK (1.4).The PopupMenuEvent will from the JPopupMenu will be forwarded to the JComboBox. A client of the JComboBox can register itself as a PopupMenuListener to receive the notification.
So JComboBox should send drop down events when the drop down popup is visible as well as invisible. JComboBox.addPopupMenuListener(PopupMenuListener) can be used to register for pop up events.

Testing:
Tested using Mach5 in all platforms available(macos,linux and windows) and got all pass.

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Commit messages:
 - 8283507: Create a regression test for RFE 4287690

Changes: https://git.openjdk.java.net/jdk/pull/7906/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7906&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283507
  Stats: 205 lines in 1 file changed: 205 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7906.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7906/head:pull/7906

PR: https://git.openjdk.java.net/jdk/pull/7906