add comment about preprocessors in tests

tests/conftest.py

@@ -8,6 +8,8 @@
 def RE() -> RunEngine:
     get_run_engine.cache_clear()
     re =  get_run_engine()
+    # Clear the preprocessors as the file writing one relies
+    # on a real signal which won't exist during testing
     re.preprocessors.clear()
 
     return re