Add tests for QA test selectors sanity scenario

qa/qa/scenario/test/sanity/selectors.rb

@@ -15,7 +15,7 @@ module QA
             validators.map(&:errors).flatten.tap do |errors|
               break if errors.none?
 
-              STDERR.puts <<~EOS
+              $stderr.puts <<~EOS
                 GitLab QA sanity selectors validation test detected problems
                 with your merge request!
 
@@ -40,7 +40,7 @@ module QA
 
               EOS
 
-              STDERR.puts errors
+              $stderr.puts errors
             end
 
             validators.each(&:validate!)

qa/spec/scenario/test/sanity/selectors_spec.rb

@@ -0,0 +1,40 @@
+describe QA::Scenario::Test::Sanity::Selectors do
+  let(:validator) { spy('validator') }
+
+  before do
+    stub_const('QA::Page::Validator', validator)
+  end
+
+  context 'when there are errors detected' do
+    before do
+      allow(validator).to receive(:errors).and_return(['some error'])
+    end
+
+    it 'outputs information about errors' do
+      expect { described_class.perform }
+        .to output(/some error/).to_stderr
+
+      expect { described_class.perform }
+        .to output(/electors validation test detected problems/)
+        .to_stderr
+    end
+  end
+
+  context 'when there are no errors detected' do
+    before do
+      allow(validator).to receive(:errors).and_return([])
+    end
+
+    it 'processes pages module' do
+      described_class.perform
+
+      expect(validator).to have_received(:new).with(QA::Page)
+    end
+
+    it 'triggers validation' do
+      described_class.perform
+
+      expect(validator).to have_received(:validate!)
+    end
+  end
+end