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.

tests/app/v2/test_errors.py

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