Fix spec/features/instance_statistics/cohorts_spec.rb

If one spec disables the usage ping in the in-memory application settings
(e.g. spec/features/usage_stats_consent_spec.rb), then this spec will
fail. To work around this problem, we enable the usage ping explicitly
in the setup phase.

Backport of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7290

spec/features/instance_statistics/cohorts_spec.rb

@@ -3,6 +3,8 @@ require 'rails_helper'
 describe 'Cohorts page' do
   before do
     sign_in(create(:admin))
+
+    stub_application_setting(usage_ping_enabled: true)
   end
 
   it 'See users count per month' do