test/ruby/test_io.rb: use IO#nonblock=

* test/ruby/test_io.rb (test_readpartial_locktmp): use IO#nonblock=
  Old fcntl invocation may drop necessary flags on some platforms.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47423 b2dd03c8-39d4-4d8f-98ff-823fe69b080e