Add a controller test for the generate_csv action

While we've already got a system test for the same purpose, we're going
to add another controller test for this action, so we're now writing a
test for the "happy path" scenario.

spec/controllers/admin/emails_download_controller_spec.rb

@@ -0,0 +1,19 @@
+require "rails_helper"
+
+describe Admin::EmailsDownloadController do
+  before do
+    admin = create(:administrator, user: create(:user, email: "admin@consul.dev"))
+    sign_in(admin.user)
+  end
+
+  describe "GET generate_csv" do
+    it "sends a list of emails in a comma-separated format" do
+      create(:user, email: "user@consul.dev")
+
+      get :generate_csv, params: { users_segment: "all_users" }
+
+      expect(response).to be_successful
+      expect(response.body).to eq "admin@consul.dev,user@consul.dev"
+    end
+  end
+end