Add tests for external_wiki_service test

This checks that the execute method does what we expect it to do.

app/models/project_services/external_wiki_service.rb

@@ -19,14 +19,19 @@ class ExternalWikiService < Service
 
   def fields
     [
-      { type: 'text', name: 'external_wiki_url', placeholder: s_('ExternalWikiService|The URL of the external Wiki'), required: true }
+      {
+        type: 'text',
+        name: 'external_wiki_url',
+        placeholder: s_('ExternalWikiService|The URL of the external Wiki'),
+        required: true
+      }
     ]
   end
 
   def execute(_data)
     @response = Gitlab::HTTP.get(properties['external_wiki_url'], verify: true) rescue nil
 
-    @response if @reponse == 200
+    @response.body if @response.code == 200
   end
 
   def self.supported_events

spec/models/project_services/external_wiki_service_spec.rb

@@ -26,4 +26,34 @@ describe ExternalWikiService do
       it { is_expected.not_to validate_presence_of(:external_wiki_url) }
     end
   end
+
+  describe 'test' do
+    before do
+      subject.properties['external_wiki_url'] = url
+    end
+
+    let(:url) { 'http://foo' }
+    let(:data) { nil }
+    let(:result) { subject.test(data) }
+
+    context 'the URL is not reachable' do
+      before do
+        WebMock.stub_request(:get, url).to_return(status: 404, body: 'not a page')
+      end
+
+      it 'is not successful' do
+        expect(result[:success]).to be_falsey
+      end
+    end
+
+    context 'the URL is reachable' do
+      before do
+        WebMock.stub_request(:get, url).to_return(status: 200, body: 'foo')
+      end
+
+      it 'is successful' do
+        expect(result[:success]).to be_truthy
+      end
+    end
+  end
 end