Write a test to verify setAccelerator method of JMenuitem.

This testcase verifies that selecting an item in the List using mouse or keys generates ItemEvent/ActionEvent appropriately.

a. Single-click on an item in the list generates ItemEvent, and double-click on an item generates ActionEvent.
b. UP/DOWN keys on an generate ItemEvent, and Enter key on an item generates ActionEvent.

Testing:
Tested using Mach5(20 times per platform) in macos,linux and windows and got all pass.