Rename "app_" fixture to "notify_admin"

This naming was introduced in 2016 without explanation [1]. I find it
confusing because:

- It's reminiscent of "_app", which is a Python convention indicating
the variable is internal, so maybe avoid using it.

- It suggests there's some other "app" fixture I should be using (there
isn't, though).

The Python style guide describes using an underscore suffix to avoid
clashes with inbuilt names [1], which is sort of applicable if we need
to import the "app" module [2]. However, we can also avoid clashes by
choosing a different name, without the strange underscore.

[1]: 3b1d521c10
[2]: 78824f54fd/tests/app/main/views/test_forgot_password.py (L5)

tests/__init__.py

@@ -559,7 +559,7 @@ def single_notification_json(
 
 
 def validate_route_permission(mocker,
-                              app_,
+                              notify_admin,
                               method,
                               response_code,
                               route,
@@ -582,8 +582,8 @@ def validate_route_permission(mocker,
     mocker.patch('app.service_api_client.get_service', return_value={'data': service})
     mocker.patch('app.models.user.Users.client_method', return_value=[usr])
     mocker.patch('app.job_api_client.has_jobs', return_value=False)
-    with app_.test_request_context():
-        with app_.test_client() as client:
+    with notify_admin.test_request_context():
+        with notify_admin.test_client() as client:
             client.login(usr)
             if session:
                 with client.session_transaction() as session_: