Add the alter_config context manager for testing

It can temporarily change an app configuration value.

tests/helpers.py

@@ -17,6 +17,8 @@
 """Helper functions and fixtures for testing
 """
 
+from contextlib import contextmanager
+
 import pytest
 
 import calsocial
@@ -70,3 +72,22 @@ def database():
         yield db
 
         db.drop_all()
+
+
+@contextmanager
+def alter_config(app, **kwargs):
+    saved = {}
+
+    for key, value in kwargs.items():
+        if key in app.config:
+            saved[key] = app.config[key]
+
+        app.config[key] = value
+
+    yield
+
+    for key, value in kwargs.items():
+        if key in saved:
+            app.config[key] = saved[key]
+        else:
+            del app.config[key]