Fix off-by-one issue in test_stream.py

[mprib]

The frame_index is currently being updated prior to the pause loop. While this can be fixed for the test easily, the GUI appears to be coded to manage the auto-stop of the playback and the enablement/disablement of the buttons with this error in place.

I need to find the place(s) in the GUI code where this is happening and make sure that things will be triggered correctly when the frame_index is updating as it should.

[mprib]

Just documenting for posterity that this test had been previously passing due to (I suspect) a lag in the processing and the frame index getting updated vs when it was checked. This did not become apparent until testing on MacOS via github actions and ran on windows just fine.