set test_errors app fixture to session scope

we have one global metrics variable `metrics = GDSMetrics()`, and we
then call `metrics.init_app` from within the flask application set up.
The v2/test_errors.py app_for_test fixture calls create_app, would call
metrics.init_app multiple times for the same metrics instance. This
causes errors, so change the fixture to session level so it only calls
once per test run.

app/__init__.py

@@ -368,9 +368,9 @@ def setup_sqlalchemy_events(app):
             # this will overwrite any previous checkout_at timestamp
             connection_record.info['checkout_at'] = time.monotonic()
 
-            # checkin runs after the request is already torn down so we'll need to capture request info earlier.
+            # checkin runs after the request is already torn down, therefore we add the request_data onto the
-            # checkout runs when the connection is first used, ie: when we first make a query, so within the request or
+            # connection_record as otherwise it won't have that information when checkin actually runs.
-            # task
+            # Note: this is not a problem for checkouts as the checkout always happens within a web request or task
 
             # web requests
             if has_request_context():

tests/app/v2/test_errors.py

@@ -3,8 +3,8 @@ from flask import url_for
 from sqlalchemy.exc import DataError
 
 
-@pytest.fixture(scope='function')
+@pytest.fixture(scope='session')
-def app_for_test(mocker):
+def app_for_test():
     import flask
     from flask import Blueprint
     from app.authentication.auth import AuthError