Extract idempotent constant into a `let`

So we can use a shorter name within specs.

spec/support/shared_examples/workers/idempotency_shared_examples.rb

@@ -16,6 +16,8 @@
 #   end
 #
 RSpec.shared_examples 'an idempotent worker' do
+  let(:worker_exec_times) { IdempotentWorkerHelper::WORKER_EXEC_TIMES }
+
   # Avoid stubbing calls for a more accurate run.
   subject do
     defined?(job_args) ? perform_multiple(job_args) : perform_multiple

spec/workers/expire_job_cache_worker_spec.rb

@@ -21,12 +21,12 @@ describe ExpireJobCacheWorker do
           allow(Gitlab::EtagCaching::Store).to receive(:new) { spy_store }
 
           expect(spy_store).to receive(:touch)
-            .exactly(IdempotentWorkerHelper::WORKER_EXEC_TIMES).times
+            .exactly(worker_exec_times).times
             .with(pipeline_path)
             .and_call_original
 
           expect(spy_store).to receive(:touch)
-            .exactly(IdempotentWorkerHelper::WORKER_EXEC_TIMES).times
+            .exactly(worker_exec_times).times
             .with(job_path)
             .and_call_original