add "unknown" status (via #523)

Allure report supports this status as well as the other ones.

Author: janza

allure-python-commons/src/model2.py

@@ -105,3 +105,4 @@ class Status(object):
     BROKEN = 'broken'
     PASSED = 'passed'
     SKIPPED = 'skipped'
+    UNKNOWN = 'unknown'