Add common ThreadingHelper module for specs (#29116)

spec/lib/request_pool_spec.rb

@@ -33,18 +33,14 @@ describe RequestPool do
 
       subject
 
-      threads = Array.new(5) do
-        Thread.new do
-          subject.with('http://example.com') do |http_client|
-            http_client.get('/').flush
-            # Nudge scheduler to yield and exercise the full pool
-            sleep(0.01)
-          end
+      multi_threaded_execution(5) do
+        subject.with('http://example.com') do |http_client|
+          http_client.get('/').flush
+          # Nudge scheduler to yield and exercise the full pool
+          sleep(0.01)
         end
       end
 
-      threads.map(&:join)
-
       expect(subject.size).to be > 1
     end