Run ExUnit Tests In A Deterministic Order

When running a file of ExUnit tests, they will be executed in a pseudo-random order based on a seed value. In general, ExUnit will pick a random seed each time you run your tests. If you'd like to add some determinism to the order that your tests run in, you can specify the seed in the ExUnit configuration.

ExUnit.configure seed: 42
ExUnit.start

defmodule AssertionTest do
  use ExUnit.Case, async: true

  test "the truth" do
    assert true
  end

  test "more truth" do
    assert 2 + 2 = 4
  end
end

source

h/t Chris Erin