Add tests for export_csv admin plans controller action
user
```ruby
# frozen_string_literal: true

require "test_helper"

module GobiertoAdmin
  module GobiertoPlans
    class PlansControllerTest < GobiertoControllerTest

      def admin
        @admin ||= gobierto_admin_admins(:nick)
      end

      def site
        @site ||= sites(:madrid)
      end

      def plan
        @plan ||= gobierto_plans_plans(:strategic_plan)
      end

      def empty_plan
        @empty_plan ||= gobierto_plans_plans(:government_plan)
      end

      def plan_with_projects_with_blank_status
        @plan_with_projects_with_blank_status ||= gobierto_plans_plans(:economic_plan)
      end

      def project
        @project ||= gobierto_plans_nodes(:political_agendas)
      end

      def test_export_csv
        with(site: site, admin: admin) do
          get admin_plans_plan_export_csv_url(plan)

          assert_response :success
          assert_equal "text/csv", response.content_type

          parsed_response = CSV.parse(response.body, headers: true)
          assert_equal 2, parsed_response.length
          last_row = parsed_response[1]

          assert_equal project.categories.first.parent_term.parent_term.name, last_row["Level 0"]
          assert_equal project.categories.first.parent_term.name, last_row["Level 1"]
          assert_equal project.categories.first.name, last_row["Level 2"]
          assert_equal project.name, last_row["Node.Title"]
          assert_equal project.status.name, last_row["Node.Status"]
          assert_equal project.progress, last_row["Node.Progress"].to_f
          assert_equal project.starts_at, Date.parse(last_row["Node.Start"])
          assert_equal project.ends_at, Date.parse(last_row["Node.End"])
          assert_equal project.options["objetivos"], last_row["Node.objetivos"]
          assert_equal project.options["temporality"], last_row["Node.temporality"]
        end
      end

      def test_export_csv_empty_plan
        with(site: site, admin: admin) do
          get admin_plans_plan_export_csv_url(empty_plan)

          assert_response :success
          assert_equal "text/csv", response.content_type

          parsed_response = CSV.parse(response.body, headers: true)
          assert_equal 3, parsed_response.length
          parsed_response.each do |row|
            assert_nil row["Node.Progress"]
          end
        end
      end

      def test_export_csv_plan_with_projects_with_blank_status
        with(site: site, admin: admin) do
          get admin_plans_plan_export_csv_url(plan_with_projects_with_blank_status)

          assert_response :success
          assert_equal "text/csv", response.content_type

          parsed_response = CSV.parse(response.body, headers: true)
          assert_equal 1, parsed_response.length
          assert_equal "0.0", parsed_response[0]["Node.Progress"]
        end
      end

    end
  end
end
```
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