increase additional nfiles

On recent my environment, it must be more than 11.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52760 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_io.rb

@@ -3081,7 +3081,7 @@ End
 
       # try to raise RLIM_NOFILE to >FD_SETSIZE
       begin
-        Process.setrlimit(Process::RLIMIT_NOFILE, fd_setsize+10)
+        Process.setrlimit(Process::RLIMIT_NOFILE, fd_setsize+20)
       rescue Errno::EPERM
         exit 0
       end