You can access the test database by using MIX_ENV=test followed by a command such as mix do ecto.drop, mix ecto.reset or mix ecto.rollback.
In this particular case, I used:
MIX_ENV=test mix ecto.reset
If your application has multiple repos (DBs), you’ll want to specify a specific repo to avoid applying the operation to all repos. For example
mix ecto.drop --repo Order.Repo
To find out more about an Ecto task, use mix help <task>