Add common ThreadingHelper module for specs (#29116)

spec/support/threading_helpers.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+module ThreadingHelpers
+  def multi_threaded_execution(thread_count)
+    wait_for_start = true
+
+    threads = Array.new(thread_count) do
+      Thread.new do
+        true while wait_for_start
+        yield
+      end
+    end
+
+    wait_for_start = false
+    threads.each(&:join)
+  end
+end