Explicitly enable usage ping for instance statistics spec

This is similar to https://gitlab.com/gitlab-org/gitlab-ee/issues/7511, but
the root cause is https://gitlab.com/gitlab-org/gitlab-ce/issues/51268.

spec/support/shared_examples/instance_statistics_controllers_shared_examples.rb

@@ -5,6 +5,8 @@ shared_examples 'instance statistics availability' do
 
   before do
     sign_in(user)
+
+    stub_application_setting(usage_ping_enabled: true)
   end
 
   describe 'GET #index' do