How to test an API client in Elixir?

mybank.ex

defmodule MyBank.Client do
  defstruct client_id: nil, access_token: nil
  
  def request(client, method, path, params \\ %{}, body \\ nil, headers \\ []) do
    # prepare url, headers, etc.
    case HTTPoison.request(method, url, body, headers) do
      {:ok, %HTTPoison.Response{status_code: 200, body: body}} -> {:ok, body}
      # handle other cases ...
    end
  end
end

defmodule MyBank.Accounts do
  def list(client) do
    case MyBank.Client.request(client, :get, "/accounts") do
      {:ok, body} -> Poison.decode!(body, as: %{"accounts" => [MyBank.Account])
      {:
    end
  end
end