In order to call the seeds from inside test_helper.exs, one option is to move the seeds file into a separate module:

defmodule MyApp.Seeds do
  def call do
    # Seed actions
  end
end

Then MyApp.Seeds.call can easily be called from either priv/repo/seeds.exs or test/test_helper.exs.

Thanks to Ihor Katkov in the #ecto Slack channel for proposing the solution.

Another way to do it currently is to call add run path/to/test_seeds.exs to the test alias in the projects mix.exs:

  defp aliases do
    [
      "ecto.setup": ["ecto.create", "ecto.migrate", "run priv/repo/seeds.exs"],
      "ecto.reset": ["ecto.drop", "ecto.setup"],
      "test": ["ecto.create --quiet", "ecto.migrate", "run test/test_seeds.exs", "test"]
    ]
  end

One thing to keep in mind though is that the seeds are run (of course) every time mix test is invoked, so the seeds will probably need to have some kind of conditional logic in place to only insert items in case they do not exist yet, something like this for example:

alias Foo.Category
alias Foo.Repo

if !Repo.get_by(Category, name: "fascinating") do
  Repo.insert(%Category{name: "fascinating"})
end

Then again, it might be more appropriate to insert the needed database records within the tests itself (or case file), which might make the tests a bit more explicit and self-contained and no conditional logic would be needed because tests are run within a transaction/cleaned up automatically.

Thanks Michał.