Add shared example for Expireable concern (#33369)

spec/models/mute_spec.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe Mute do
+  include_examples 'Expireable'
+
+  describe 'Associations' do
+    it { is_expected.to belong_to(:account).required }
+    it { is_expected.to belong_to(:target_account).required }
+  end
+
+  describe 'Validations' do
+    subject { Fabricate.build :mute }
+
+    it { is_expected.to validate_uniqueness_of(:account_id).scoped_to(:target_account_id) }
+  end
+end