tests: define load_tests otherwise doctest are not run in software/slapos-testing

When we switch to using `python -m unittest discover -v` test_suite is not
used, but load_tests is used instead.

slapos/recipe/cmmi/tests.py

@@ -412,3 +412,7 @@ def test_suite():
         ]
     suite = unittest.TestSuite(tests)
     return suite
+
+
+def load_tests(*args):
+  return test_suite()