As explained at
https://hexdocs.pm/ecto_sqlite3/Ecto.Adapters.SQLite3.html#module-async-sandbox-testing:
The Ecto SQLite3 adapter does not support async tests when used with
Ecto.Adapters.SQL.Sandbox. This is due to SQLite only allowing up one
write transaction at a time, which often does not work with the Sandbox
approach of wrapping each test in a transaction.
Claudio Ortolina