Add :slow tag to example specs for filter by tag

Use "bin/verify --tag ~@slow" to exclude tests tagged as :slow.

Author: rossta

spec/system/examples_spec.rb

@@ -1,6 +1,6 @@
 require "rails_helper"
 
-RSpec.xdescribe "Examples", type: :system do
+RSpec.describe "Examples", :slow, type: :system do
   it "shows an example" do
     visit example_path("hello_ruby")