You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: test_mutex.rb
    path of file: ./repos/jruby/test/mri/fiber
    
    the code of the file until where you have to start completion: # frozen_string_literal: true
require 'test/unit'
require_relative 'scheduler'

class TestFiberMutex < Test::Unit::TestCase
  def
    
  def test_mutex_thread
    mutex = Thread::Mutex.new
    mutex.lock

    thread = Thread.new do
      scheduler = Scheduler.new
      Fiber.set_scheduler scheduler

      Fiber.schedule do
        mutex.lock
        sleep 0.1
        mutex.unlock
      end

      scheduler.run
    end

    sleep 0.1
    mutex.unlock

    thread.join
  end