test_io.rb: duplicated test

* test/ruby/test_io.rb (test_s_write): merge duplicated test.
* test/ruby/test_io.rb (test_io_s_write): remove duplicated
  assertsions


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44272 b2dd03c8-39d4-4d8f-98ff-823fe69b080e