Add unit tests for bioxel module and documentation

- Created README.md for test instructions and categories.
- Implemented comprehensive unit tests for the Data, Layer, and utility functions in the bioxel module.
- Added tests for reading NRRD and DICOM formats, data properties, and edge cases.
- Included tests for real data files to ensure functionality with actual datasets.

AGENTS.md

@@ -24,6 +24,11 @@ uv sync
 uv run build.py <platform>  # platforms: windows-x64, linux-x64, macos-arm64, macos-x64
 ```
 
+### Unit Test
+```bash
+uv run pytest tests/
+```
+
 ### Code Quality
 ```bash
 # Format code (autopep8)