Add common ThreadingHelper module for specs (#29116)

spec/models/account_spec.rb

@@ -1035,19 +1035,10 @@ RSpec.describe Account do
     it 'increments the count in multi-threaded an environment when account_stat is not yet initialized' do
       subject
 
-      increment_by   = 15
-      wait_for_start = true
-
-      threads = Array.new(increment_by) do
-        Thread.new do
-          true while wait_for_start
-          described_class.find(subject.id).increment_count!(:followers_count)
-        end
+      multi_threaded_execution(15) do
+        described_class.find(subject.id).increment_count!(:followers_count)
       end
 
-      wait_for_start = false
-      threads.each(&:join)
-
       expect(subject.reload.followers_count).to eq 15
     end
   end