* Tests: Add tests for addDarkness() in World.java
Add Junit tests for the addDarkness found in core/src/io/anuke/mindustry/core/World.java
* Refactor: Remove code duplication
Remove code duplication in WorldTests. Invoke launchApplication directly from ApplicationTests.
* BugFix: Assign Tiles[][] tiles used for testing.
Add assignment of Tiles[][] tiles separately when invoking ApplicationTests.launchApplication.
