test.ex

defmodule Test.Behaviour do
  @doc "Define The Behaviour"

  @callback handle_something(String.t) :: String.t
end

defmodule Test.Util do
  def handle_general_case("general_special") do
    "return something general but special"
  end

  def handle_general_case(_) do
    "return something general"
  end
end

defmodule Test.Special do
  alias Test.Util
  @behaviour Test.Behaviour

  def handle_something("special") do
    "return something special"
  end

  def handle_something(param) do
    Util.handle_general_case(param)
  end
end

test_test.exs

defmodule Test.SpecialTest do
  use ExUnit.Case, async: true

  test "General Case" do
    assert Test.Special.handle_something("general") == "return something general"
  end

  test "Special Case"  do
    assert Test.Special.handle_something("special") == "return something special"
  end

  test "General but special case" do
    assert Test.Special.handle_something("general_special") == "return something general but special"
  end
end