Fix missing mock in rack attack ee spec

ee/spec/lib/gitlab/rack_attack_spec.rb

@@ -6,6 +6,7 @@ RSpec.describe Gitlab::RackAttack, :aggregate_failures do
   describe '.configure' do
     let(:fake_rack_attack) { class_double("Rack::Attack") }
     let(:fake_rack_attack_request) { class_double("Rack::Attack::Request") }
+    let(:fake_cache) { instance_double("Rack::Attack::Cache") }
 
     before do
       stub_const("Rack::Attack", fake_rack_attack)
@@ -16,6 +17,8 @@ RSpec.describe Gitlab::RackAttack, :aggregate_failures do
       allow(fake_rack_attack).to receive(:track)
       allow(fake_rack_attack).to receive(:safelist)
       allow(fake_rack_attack).to receive(:blocklist)
+      allow(fake_rack_attack).to receive(:cache).and_return(fake_cache)
+      allow(fake_cache).to receive(:store=)
     end
 
     it 'adds the incident management throttle' do