Un-truncate test failure diffs

(cherry picked from commit de8e158ce8)

conftest.py

@@ -2,6 +2,8 @@
 Default unit test configuration and fixtures.
 """
 from __future__ import absolute_import, unicode_literals
+from unittest import TestCase
+
 import pytest
 
 # Import hooks and fixture overrides from the cms package to
@@ -9,6 +11,10 @@ import pytest
 
 from cms.conftest import _django_clear_site_cache, pytest_configure  # pylint: disable=unused-import
 
+# When using self.assertEquals, diffs are truncated. We don't want that, always
+# show the whole diff.
+TestCase.maxDiff = None
+
 
 @pytest.fixture(autouse=True)
 def no_webpack_loader(monkeypatch):