Backend: Tests — notifications, invites, members

[harshal-2304]

Scope

tests/notification/test_notification_service.py, test_notification_views.py
tests/plans/groups/test_groups_service.py, test_groups_views.py, test_group_invite_email.py, test_invite_expiry_job.py
tests/db/test_mongo_database_lifespan.py
PR: #661