test_io.rb: ignore stream closed IOError

* test/ruby/test_io.rb (test_readpartial_locktmp): stream closed
  IOError while reading is not a matter.  [Bug #10193]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47353 b2dd03c8-39d4-4d8f-98ff-823fe69b080e