dev.exs

config :my_app, :twitter_api,
  client: Twitter.SandboxClient

test.exs

config :my_app, :twitter_api,
  client: Twitter.TestClient

timeline_controller.ex

def index(conn, %{"username" => username}) do
  client = Application.get_env(:my_app, :twitter_api)[:client]
  
  client.get(username)
end

Yes! You can configure it directly too if you prefer:

config :my_app, :twitter_api, Twitter.SandboxClient

And in your controller I would move it to a function:

def index(conn, %{"username" => username}) do
  twitter_api.get(username)
end

defp twitter_api do
  Application.get_env(:my_app, :twitter_api)
end