Remove execution_time from the results_bag fixture and use 'duration_ms' instead of 'duration' in pytest info

[smarie]

It seems that the best duration estimate is the one from pytest because it is really made right before/after the call. However it is not clear that the number was in seconds, so it is probably preferred to have a number of milliseconds and an explicit name.

I will open a separate issue on pytest concerning the way these durations are measured by pytest.