Add config overrides steps to pytest

This requires some patching around the config object, which now happens
in every test.

Co-authored-by: Micah Jerome Ellison <micah.jerome.ellison@gmail.com>

tests/lib/fixtures.py

@@ -143,10 +143,15 @@ def user_input():
 
 
 @fixture
-def config_data(config_path):
+def config_on_disk(config_path):
     return load_config(config_path)
 
 
+@fixture
+def config_in_memory():
+    return dict()
+
+
 @fixture
 def journal_name():
     return None