Python test: new options provide local test data paths

New options:
- `--repo` - path to OpenCV repository, script will look for some files there (_lena.jpg_, etc.).
- `--data` - path to other data folder, script will search for additional files there (_camera_calibration.tar.gz_).

Example (assuming repository is in `../opencv` folder):
```
python ../opencv/modules/python/test/test.py --data ~/Downloads --repo ../opencv
```
If local file is not found, script will download it from the internet (current behaviour).
Buildbot support will be added later.