Add spec for ReferenceExtractor#labels

spec/lib/gitlab/reference_extractor_spec.rb

@@ -106,6 +106,15 @@ describe Gitlab::ReferenceExtractor do
     expect(subject.merge_requests).to eq([@m1, @m0])
   end
 
+  it 'accesses valid labels' do
+    @l0 = create(:label, title: 'one', project: project)
+    @l1 = create(:label, title: 'two', project: project)
+    @l2 = create(:label)
+
+    subject.analyze("~#{@l0.id}, ~999, ~#{@l2.id}, ~#{@l1.id}")
+    expect(subject.labels).to eq([@l0, @l1])
+  end
+
   it 'accesses valid snippets' do
     @s0 = create(:project_snippet, project: project)
     @s1 = create(:project_snippet, project: project)