remove usage of notify_db fixture in unit tests

* notify_db fixture creates the database connection and ensures the test
  db exists and has migrations applied etc. It will run once per session
  (test run).
* notify_db_session fixture runs after your test finishes and deletes
  all non static (eg type table) data.

In unit tests that hit the database (ie: most of them), 99% of the time
we will need to use notify_db_session to ensure everything is reset. The
only time we don't need to use it is when we're querying things such as
"ensure get X works when database is empty". This is such a low
percentage of tests that it's easier for us to just use
notify_db_session every time, and ensure that all our tests run much
more consistently, at the cost of a small bit of performance when
running tests.

We used to use notify_db to access the session object for manually
adding, committing, etc. To dissuade usage of that fixture I've moved
that to the `notify_db_session`. I've then removed all uses of notify_db
that I could find in the codebase.

As a note, if you're writing a test that uses a `sample_x` fixture, all
of those fixtures rely on notify_db_session so you'll get the teardown
functionality for free. If you're just calling eg `create_x` db.py
functions, then you'll need to make you add notify_db_session fixture to
your test, even if you aren't manually accessing the session.

tests/app/conftest.py

@@ -526,7 +526,7 @@ def sample_email_notification(notify_db_session):
 
 
 @pytest.fixture(scope='function')
-def sample_notification_history(notify_db, notify_db_session, sample_template):
+def sample_notification_history(notify_db_session, sample_template):
     created_at = datetime.utcnow()
     sent_at = datetime.utcnow()
     notification_type = sample_template.template_type
@@ -545,8 +545,8 @@ def sample_notification_history(notify_db, notify_db_session, sample_template):
         api_key_id=api_key and api_key.id,
         sent_at=sent_at
     )
-    notify_db.session.add(notification_history)
-    notify_db.session.commit()
+    notify_db_session.add(notification_history)
+    notify_db_session.commit()
 
     return notification_history
 
@@ -852,7 +852,7 @@ def letter_volumes_email_template(notify_service):
 
 
 @pytest.fixture
-def notify_service(sample_user):
+def notify_service(notify_db_session, sample_user):
     service = Service.query.get(current_app.config['NOTIFY_SERVICE_ID'])
     if not service:
         service = Service(
@@ -876,19 +876,19 @@ def notify_service(sample_user):
         }
         reply_to = ServiceEmailReplyTo(**data)
 
-        db.session.add(reply_to)
-        db.session.commit()
+        notify_db_session.add(reply_to)
+        notify_db_session.commit()
 
     return service
 
 
 @pytest.fixture(scope='function')
-def sample_service_guest_list(notify_db, notify_db_session):
+def sample_service_guest_list(notify_db_session):
     service = create_service(check_if_service_exists=True)
     guest_list_user = ServiceGuestList.from_string(service.id, EMAIL_TYPE, 'guest_list_user@digital.gov.uk')
 
-    notify_db.session.add(guest_list_user)
-    notify_db.session.commit()
+    notify_db_session.add(guest_list_user)
+    notify_db_session.commit()
     return guest_list_user
 
 
@@ -933,7 +933,7 @@ def nhs_email_branding(notify_db_session):
 
 
 @pytest.fixture
-def restore_provider_details(notify_db, notify_db_session):
+def restore_provider_details(notify_db_session):
     """
     We view ProviderDetails as a static in notify_db_session, since we don't modify it... except we do, we updated
     priority. This fixture is designed to be used in tests that will knowingly touch provider details, to restore them
@@ -955,10 +955,10 @@ def restore_provider_details(notify_db, notify_db_session):
     # also delete these as they depend on provider_details
     ProviderDetails.query.delete()
     ProviderDetailsHistory.query.delete()
-    notify_db.session.commit()
-    notify_db.session.add_all(existing_provider_details)
-    notify_db.session.add_all(existing_provider_details_history)
-    notify_db.session.commit()
+    notify_db_session.commit()
+    notify_db_session.add_all(existing_provider_details)
+    notify_db_session.add_all(existing_provider_details_history)
+    notify_db_session.commit()
 
 
 @pytest.fixture