message: python(test): add tests filter
  Usage example (bash):
  $ OPENCV_PYTEST_FILTER=test_digits python test.py -v
diff: @@ -17,7 +17,7 @@ from tests_common import NewOpenCVTests
  basedir = os.path.abspath(os.path.dirname(__file__))
  def load_tests(loader, tests, pattern):
  - tests.addTests(loader.discover(basedir, pattern='test_*.py'))
  + tests.addTests(loader.discover(basedir, pattern=os.environ.get('OPENCV_PYTEST_FILTER', 'test_') + '*.py'))
  return tests
  if __name__ == '__main__':
  