restore timeout

test/ruby/test_fiber.rb

@@ -349,9 +349,7 @@ class TestFiber < Test::Unit::TestCase
     env['RUBY_FIBER_VM_STACK_SIZE'] = vm_stack_size.to_s if vm_stack_size
     env['RUBY_FIBER_MACHINE_STACK_SIZE'] = machine_stack_size.to_s if machine_stack_size
     out = Dir.mktmpdir("test_fiber") {|tmpdir|
-      out, err, status = EnvUtil.invoke_ruby([env, '-e', script], '', true, true, chdir: tmpdir,
-        # TODO: this 6 hours timeout is to debug an issue on MacOS.
-        timeout: 60 * 60 * 6)
+      out, err, status = EnvUtil.invoke_ruby([env, '-e', script], '', true, true, chdir: tmpdir, timeout: 30)
       assert(!status.signaled?, FailDesc[status, nil, err])
       out
     }