Add a new `stub_mattermost_setting` stub helper to properly stub Mattermost settings

Signed-off-by: Rémy Coutable <remy@rymai.me>

spec/features/groups_spec.rb

@@ -46,7 +46,7 @@ feature 'Group', feature: true do
 
     describe 'Mattermost team creation' do
       before do
-        allow(Settings.mattermost).to receive_messages(enabled: mattermost_enabled)
+        stub_mattermost_setting(enabled: mattermost_enabled)
 
         visit new_group_path
       end

spec/features/projects/services/mattermost_slash_command_spec.rb

@@ -7,7 +7,7 @@ feature 'Setup Mattermost slash commands', :feature, :js do
   let(:mattermost_enabled) { true }
 
   before do
-    Settings.mattermost['enabled'] = mattermost_enabled
+    stub_mattermost_setting(enabled: mattermost_enabled)
     project.team << [user, :master]
     login_as(user)
     visit edit_namespace_project_service_path(project.namespace, project, service)

spec/services/groups/create_service_spec.rb

@@ -44,7 +44,7 @@ describe Groups::CreateService, '#execute', services: true do
     let!(:service) { described_class.new(user, params) }
 
     before do
-      Settings.mattermost['enabled'] = true
+      stub_mattermost_setting(enabled: true)
     end
 
     it 'create the chat team with the group' do

spec/support/stub_configuration.rb

@@ -21,6 +21,10 @@ module StubConfiguration
     allow(Gitlab.config.incoming_email).to receive_messages(messages)
   end
 
+  def stub_mattermost_setting(messages)
+    allow(Gitlab.config.mattermost).to receive_messages(messages)
+  end
+
   private
 
   # Modifies stubbed messages to also stub possible predicate versions