Refactor tests into package structure mirroring source code

- Moved tests to `tests/ndi/unittest/...`
- Renamed test files to match module names.
- Updated README.md with correct test execution command.
- Removed `tests/ndi/__init__.py` to prevent namespace shadowing.

Author: google-labs-jules[bot]

README.md

@@ -46,7 +46,7 @@ pip install -e .
 To run the tests, use the following command:
 
 ```bash
-python -m unittest discover -s tests -t .
+python -m unittest discover -s tests/ndi/unittest -t .
 ```
 
 ### Building Documentation