If black fails, add diff output

Signed-off-by: Pablo Barbáchano <[email protected]>

Author: pb8o

tests/integration_tests/style/test_python.py

@@ -12,4 +12,4 @@ def test_python_style():
     @type: style
     """
     # Runs command
-    utils.run_cmd("black . --check")
+    utils.run_cmd("black . --check --diff")