Add more test cases for Ambient Light Events

- Add idlharness.html using idlharness.js
- Add LightLevelEvent_tests.js for LightLevelEvent interface
- Rename DeviceLight_tests.js to DeviceLightEvent_tests.js
  and update it a little
- Rename DeviceLight_tests.html to AmbientLight_tests.html
  to include LightLevelEvent_tests.js