use a contextmanager to set config values

so that when later tests run, the config has been restored to its
defaults and we don't end up failing later tests in confusing ways

tests/conftest.py

@@ -1,3 +1,4 @@
+from contextlib import contextmanager
 import os
 
 import boto3
@@ -83,3 +84,11 @@ def pytest_generate_tests(metafunc):
         argnames, testdata = idparametrize.args
         ids, argvalues = zip(*sorted(testdata.items()))
         metafunc.parametrize(argnames, argvalues, ids=ids)
+
+
+@contextmanager
+def set_config(app, name, value):
+    old_val = app.config.get(name)
+    app.config[name] = value
+    yield
+    app.config[name] = old_val