Add specs for Milestone reference methods

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

spec/models/milestone_spec.rb

@@ -519,4 +519,20 @@ describe Milestone do
       end
     end
   end
+
+  describe '.reference_pattern' do
+    subject { described_class.reference_pattern }
+
+    it { is_expected.to match('gitlab-org/gitlab-ce%123') }
+    it { is_expected.to match('gitlab-org/gitlab-ce%"my-milestone"') }
+  end
+
+  describe '.link_reference_pattern' do
+    subject { described_class.link_reference_pattern }
+
+    it { is_expected.to match("#{Gitlab.config.gitlab.url}/gitlab-org/gitlab-ce/milestones/123") }
+    it { is_expected.to match("#{Gitlab.config.gitlab.url}/gitlab-org/gitlab-ce/-/milestones/123") }
+    it { is_expected.not_to match("#{Gitlab.config.gitlab.url}/gitlab-org/gitlab-ce/issues/123") }
+    it { is_expected.not_to match("gitlab-org/gitlab-ce/milestones/123") }
+  end
 end