pytest.ini: Enable strict xfail

Tests marked as xfailing should be failing - if they are not, it means
something in Scylla changed and we need to adapt the tests.
This attribute means that a passing xtest will cause a failure.

Author: Lorak-mmk

pytest.ini

@@ -1,4 +1,5 @@
 [pytest]
 log_format = %(asctime)s.%(msecs)03d %(levelname)s [%(module)s:%(lineno)s]: %(message)s
 log_level = DEBUG
-log_date_format = %Y-%m-%d %H:%M:%S
+log_date_format = %Y-%m-%d %H:%M:%S
+xfail_strict=true