stub Devise.omniauth_providers to return GitHub even if the gitlab.yml has no...

stub Devise.omniauth_providers to return GitHub even if the gitlab.yml has no omniauth provider enabled
This will fix failing tests in case gitlab.yml file has no omniauth providers enabled

spec/models/application_setting_spec.rb

@@ -20,8 +20,14 @@ describe ApplicationSetting, models: true do
     it { is_expected.to allow_value(https).for(:after_sign_out_path) }
     it { is_expected.not_to allow_value(ftp).for(:after_sign_out_path) }
 
-    it { is_expected.to allow_value([:github]).for(:disabled_oauth_sign_in_sources) }
-    it { is_expected.not_to allow_value([:test]).for(:disabled_oauth_sign_in_sources) }
+    describe 'disabled_oauth_sign_in_sources validations' do
+      before do
+        allow(Devise).to receive(:omniauth_providers).and_return([:github])
+      end
+
+      it { is_expected.to allow_value(['github']).for(:disabled_oauth_sign_in_sources) }
+      it { is_expected.not_to allow_value(['test']).for(:disabled_oauth_sign_in_sources) }
+    end
 
     it { is_expected.to validate_presence_of(:max_attachment_size) }