tests: fix counting selenium failures

1b1dbf60 (tests: also consider python unittest failures in functional tests, 2021-06-16) was not counting properly the cases where we have selenium failures. In that case we only want to count selenium failures, if we add with the python failures from status_dict, we report one extra failures.

The correct approach is to count selenium failures if any and otherwise count python failures

Co-authored-by: Vincent Pelletier vincent@nexedi.com