Update BackOfficeTestCase

- Rename `self.user` to `self.admin` and `self.conference_member` to
  `self.admin_member` to better reflect the purpose of the objects.
- Add `self.user` and `self.user_cm` to create a non-staff user and
  associate it with the conference.

src/backoffice/tests/base.py

@@ -25,10 +25,15 @@ class BackOfficeTestCase(TestCase):
             is_public=True,
         )
         self.conf.save()
-        self.user = PlatformUser(username='testuser', email='no@where.test', is_staff=True, is_superuser=True)
-        self.user.save()
-        self.conference_member = ConferenceMember(conference=self.conf, user=self.user)
-        self.conference_member.save()
+        self.admin = PlatformUser.objects.create(
+            username='test_admin',
+            email='no@where.test',
+            is_staff=True,
+            is_superuser=True,
+        )
+        self.admin_member = ConferenceMember.objects.create(conference=self.conf, user=self.admin)
+        self.user = PlatformUser.objects.create(username='test_user', email='no@where.test')
+        self.user_cm = ConferenceMember.objects.create(conference=self.conf, user=self.user)
         self.staff = PlatformUser.objects.create(username='test_staff', email='staff@where.test')
         self.staff_cm = ConferenceMember.objects.create(conference=self.conf, user=self.staff, is_staff=True)
         self.staff_cm.permission_groups.add(Group.objects.get(name='Assembly-Team'))