[refactor] move tests to tests/, add pytest functions and main() entrypoints

- Moved src/tests/ → tests/
- Added test_* functions with assertions to script-style test files
- Added main() to each so IDEs offer it as a separate run target from pytest
- Fixed cupy_test.py: remove spurious x_gpu += x_cpu, fix duplicate xlabel

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -6,5 +6,4 @@ images
 *.egg-info
 __pycache__
 *.npzdata
-.ipynb_checkpoints/
-cupy_test.py
+.ipynb_checkpoints/