tests: add related tags controller tests.

test/functional/related_tags_controller_test.rb

@@ -1,7 +1,12 @@
 require 'test_helper'
 
 class RelatedTagsControllerTest < ActionController::TestCase
-  # test "the truth" do
-  #   assert true
-  # end
+  context "The related tags controller" do
+    context "show action" do
+      should "work" do
+        get :show, { query: "touhou" }
+        assert_response :success
+      end
+    end
+  end
 end