Tests: use mutex with multitthreaded Ruby hooks.

This commit fixes a rare crash that can occur when File.write is
called by many threads.

test/ruby/hooks/multiple.rb

@@ -1,9 +1,13 @@
 require 'securerandom'
 
+@mutex = Mutex.new
+
 on_worker_boot do
     File.write("./cookie_worker_boot.#{SecureRandom.hex}", "worker booted")
 end
 
 on_thread_boot do
-    File.write("./cookie_thread_boot.#{SecureRandom.hex}", "thread booted")
+    @mutex.synchronize do
+        File.write("./cookie_thread_boot.#{SecureRandom.hex}", "thread booted")
+    end
 end