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/app/notify_client/test_notify_admin_api_client.py

@@ -28,10 +28,10 @@ from tests.conftest import (
     service_json(active=True),
     None
 ], ids=['active_service', 'no_service'])
-def test_active_service_can_be_modified(app_, method, user, service):
+def test_active_service_can_be_modified(notify_admin, method, user, service):
     api_client = NotifyAdminAPIClient()
 
-    with app_.test_request_context() as request_context, app_.test_client() as client:
+    with notify_admin.test_request_context() as request_context, notify_admin.test_client() as client:
         client.login(user)
         request_context.service = Service(service)
 
@@ -47,10 +47,10 @@ def test_active_service_can_be_modified(app_, method, user, service):
     'post',
     'delete'
 ])
-def test_inactive_service_cannot_be_modified_by_normal_user(app_, api_user_active, method):
+def test_inactive_service_cannot_be_modified_by_normal_user(notify_admin, api_user_active, method):
     api_client = NotifyAdminAPIClient()
 
-    with app_.test_request_context() as request_context, app_.test_client() as client:
+    with notify_admin.test_request_context() as request_context, notify_admin.test_client() as client:
         client.login(api_user_active)
         request_context.service = Service(service_json(active=False))
 
@@ -66,10 +66,10 @@ def test_inactive_service_cannot_be_modified_by_normal_user(app_, api_user_activ
     'post',
     'delete'
 ])
-def test_inactive_service_can_be_modified_by_platform_admin(app_, platform_admin_user, method):
+def test_inactive_service_can_be_modified_by_platform_admin(notify_admin, platform_admin_user, method):
     api_client = NotifyAdminAPIClient()
 
-    with app_.test_request_context() as request_context, app_.test_client() as client:
+    with notify_admin.test_request_context() as request_context, notify_admin.test_client() as client:
         client.login(platform_admin_user)
         request_context.service = Service(service_json(active=False))
 
@@ -80,10 +80,10 @@ def test_inactive_service_can_be_modified_by_platform_admin(app_, platform_admin
     assert ret == request.return_value
 
 
-def test_generate_headers_sets_standard_headers(app_):
+def test_generate_headers_sets_standard_headers(notify_admin):
     api_client = NotifyAdminAPIClient()
-    with set_config(app_, 'ROUTE_SECRET_KEY_1', 'proxy-secret'):
-        api_client.init_app(app_)
+    with set_config(notify_admin, 'ROUTE_SECRET_KEY_1', 'proxy-secret'):
+        api_client.init_app(notify_admin)
 
     # with patch('app.notify_client.has_request_context', return_value=False):
     headers = api_client.generate_headers('api_token')
@@ -95,11 +95,11 @@ def test_generate_headers_sets_standard_headers(app_):
     assert headers['X-Custom-Forwarder'] == 'proxy-secret'
 
 
-def test_generate_headers_sets_request_id_if_in_request_context(app_):
+def test_generate_headers_sets_request_id_if_in_request_context(notify_admin):
     api_client = NotifyAdminAPIClient()
-    api_client.init_app(app_)
+    api_client.init_app(notify_admin)
 
-    with app_.test_request_context() as request_context:
+    with notify_admin.test_request_context() as request_context:
         headers = api_client.generate_headers('api_token')
 
     assert set(headers.keys()) == {