Fix broken test_visualization.py

Running onionperf's tests we get several instances of:

Traceback (most recent call last):
  File "/home/gk/venv/lib/python3.9/site-packages/nose/case.py", line 198, in runTest
    self.test(*self.arg)
  File "/home/gk/nethealth/onionperf/onionperf/tests/test_visualization.py", line 123, in test_create_data_frame_v3p1_no_filters
    tgen_viz._TGenVisualization__extract_data_frame()
TypeError: __extract_data_frame() missing 2 required positional arguments: 'onion' and 'public'

(triggered by test_visualization.test_create_data_frame_v3 and test_visualization.test_create_data_frame_v3p1_filters)