[Lifecycle] Set up headless testing for core logic

[stringdotjar]

Implement a testing suite using JUnit and libGDX's headless backend to verify core framework logic without requiring a GPU.

Focus areas:

• FlixelMathUtil calculations.
• FlixelObject motion and velocity integration.
• FlixelGroup member management (add/remove/sort).
• Signal dispatching and listener management.
• Anything else that may be needed.