Bug 768228 - Added test_system_messages.py for testing system message ba...

Added a test case to test system message basic functions. This test uses 'test container' as the testing app and 'dummy-message' as the tesing system message. It requires 
- gecko to add 'dummy-message' to SystemMessagePermissionCheck.jsm
- manifest.webapp of test container to add 
`` 
  "messages": [
     { "dummy-message": "/index.html" }
  ]
``