tests: include load_tests for python -m unittest discover

Since we switched to running tests with python -m unittest discover,
doctests were not longer ran.

slapos/recipe/build/tests.py

@@ -585,5 +585,10 @@ def test_suite():
       ))
   return suite
 
+
+def load_tests(*args):
+  return test_suite()
+
+
 if __name__ == '__main__':
   unittest.main(defaultTest='test_suite')