Individual migration tests define sesh_with_large_data but never use it

[rod-glover]

In tests/alembic_migrations/versions, we define tests of single migrations. These occasionally need data to test migrations that include a data migration component, but they are relatively rare. However, the fixture sesh_with_large_data is defined in the conftest for each one. This is likely unnecessary; in fact I think that defining the fixture at a higher level could replace all of them.