Add integration test

[Wojtek Mach]

Something like this:

# test/integration_test.exs
@moduletag :integration

test "it works" do
  credentials_path = System.get("GOOGLE_APPLICATION_CREDENTIALS", "credentials.json")
  project_id = System.fetch!("PROJECT_ID")

  credentials =
    System.get("GOOGLE_APPLICATION_CREDENTIALS", "credentials.json")
    |> File.read!()
    |> Jason.decode!()

  source = {:service_account, credentials, []}
  {:ok, _} = Goth.start_link(name: MyGoth, source: source, http_client: &Req.request/1)

  req =
    Req.new()
    |> ReqBigQuery.attach(project_id: project_id, dataset: "livebook", goth: MyGoth)

  result = Req.post!(req, bigquery: "SELECT sepal_length, sepal_width FROM iris limit 2").body
  assert result.num_rows == 2
  assert result.columns == ["sepal_length", "sepal_width"]
  assert [[x1, y1], [x2, y2]] = result.rows
  assert is_float(x1)
  assert is_float(y1)
  assert is_float(x2)
  assert is_float(x2)
end

by default, we wouldn't run tests tagged with integration, however we would have a mix test.all alias to include those. See how Req does it.