Add mocks for some SDL functions related to joysticks/gamepads.

Remove old SDL dependency.

Ensure input test implementation is aligned to current input API.

Separate mock library from source.

Put mocks to directory structure mimicking project structure.

Remove symlinks and make codebase use common modules instead.

Add tests for initialization and teardown for all input methods.

Use consistent integer types for mocks.

Update input tests.

Implement tests for joystick axis, hat, and button events.