get back pytest's pretty assertion failures, fixes #1938

conftest.py

@@ -4,6 +4,8 @@ import sys
 
 import pytest
 
+# needed to get pretty assertion failures in unit tests:
+pytest.register_assert_rewrite('borg')
 
 # This is a hack to fix path problems because "borg" (the package) is in the source root.
 # When importing the conftest an "import borg" can incorrectly import the borg from the