python(test): add tests filter

Usage example (bash):
$ OPENCV_PYTEST_FILTER=test_digits python test.py -v

modules/python/test/test.py

@@ -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__':