add controller tests

spec/controllers/main_controller_spec.rb

@@ -0,0 +1,11 @@
+require "rails_helper"
+
+describe MainController, type: :controller do
+
+  describe "GET #index" do
+    it "returns success" do
+      get :index
+      expect(response).to be_successful
+    end
+  end
+end