Simplify account model username presence validation spec (#31013)

spec/support/shoulda_matchers.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+Shoulda::Matchers.configure do |config|
+  config.integrate do |with|
+    with.test_framework :rspec
+    with.library :rails
+  end
+end