Add some OIDC flow tests.

Enable testing for gregui, add a django config for testing.

gregsite/settings/testing.py

+from .dev import *
+
+AUTHENTICATION_BACKENDS = [
+    "gregui.authentication.auth_backends.DevBackend",  # Fake dev backend
+    "django.contrib.auth.backends.ModelBackend",  # default
+    "gregui.authentication.auth_backends.GregOIDCBackend",
+    "sesame.backends.ModelBackend",  # link login
+]
+
+OIDC_RP_CLIENT_ID = "lalalalala"
+OIDC_RP_CLIENT_SECRET = "lalalalala"