tests: also consider python unittest failures in functional tests

It can happen that a test running selenium fail in python, but not in selenium,
like for example test_result_module/20210615-CDADEC14/183
To prevent such tests from being reported as PASS, we make the total number of
failures being the sum of the python unittest failures and the selenium failures.
2 jobs for <span class="ref-name">feat/firefox-error-log</span> in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-TestRunner2

00:39:24

failed ERP5.UnitTest-TestRunner2

04:17:19