Configure pytest for discovery of unittest_*.py files

Otherwise, only test_blog.py was run.

tox.ini

@@ -25,3 +25,6 @@ format = pylint
 max-line-length = 100
 ignore = E731,W503
 exclude = cubicweb_blog/__pkginfo__.py,cubicweb_blog/migration/*,test/data/*,.tox/*
+
+[pytest]
+python_files = *test_*.py