feat: implement basic tests (#73)

* wip: implement basic tests (no thorough check)

* fix: use proper test directory, test fix

* chore: update package-lock due to rebase

* test: add comprehensive tests for behavior coverage

- Add test for signal:true defaulting to SIGUSR2
- Add test for error when output.path is undefined
- Add tests for restartable keyboard 'rs' restart
- Add test for multiple assets logging
- Add test for keyboard option defaults
- Add test for worker without pid edge case

Coverage improved: 83% -> 98% statements, 100% functions/lines

* refactor: rebase changes

* test: strengthen weak tests with proper assertions

- Multiple assets test: now verifies correct asset selection
- NODE_ENV defaults: split into 2 tests (dev/prod) with real assertions
- Worker without pid: explicit no-throw and no-hang assertions
- Add production NODE_ENV test for keyboard default

Author: atassis