Add rspec and request tests

spec/factories/users.rb

@@ -0,0 +1,6 @@
+FactoryBot.define do
+  factory :user do
+    sequence(:username) { |n| "test-#{n.to_s.rjust(3, "0")}@sample.com" }
+    password { "12345678" }
+  end
+end